Add celery task router

Add route_task function to route celery tasks to different queues.
If we can figure out what the module build is a task ran for by
checking the task arguments, then we route this task to a queue
named:

    "mbs-{}".format(module_build_id % num_workers)

"num_workers" has default value of 1, and can be changed in
backend_config.py. If module build id can't be figured out, task will
be routed to the default queue which is named "mbs-default".

While setting up the workers, the number of workers should match with
"num_workers" in config, and each worker will listen on two queues:

    1. mbs-default
    2. mbs-{number} # for example, the first worker listens on "mbs-0"

By this design, all tasks for a particular module build will be routed
to the same queue and run on the same worker serially.

tests/test_celery_route_task.py

@@ -0,0 +1,116 @@
+# -*- coding: utf-8 -*-
+# SPDX-License-Identifier: MIT
+import mock
+
+from module_build_service import celery_app, conf
+from module_build_service.scheduler.handlers import components, greenwave, modules, repos, tags
+from module_build_service.scheduler.producer import fail_lost_builds
+
+from tests import scheduler_init_data
+
+
+@mock.patch.object(conf, "num_workers", create=True, new=3)
+@mock.patch("celery.app.amqp.AMQP.send_task_message")
+class TestCeleryRouteTask:
+    def setup_method(self, test_method):
+        self.old_task_always_eager = celery_app.conf.get("task_always_eager")
+        celery_app.conf.update(task_always_eager=False)
+
+    def teardown_method(self, test_method):
+        celery_app.conf.update(task_always_eager=self.old_task_always_eager)
+
+    def test_route_modules_init_task(self, send_task_message):
+        modules.init.delay("fakemsg", 2, 0)
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-2"
+
+    def test_route_modules_init_task_call_with_kwargs(self, send_task_message):
+        kwargs = {
+            "msg_id": "fakemsg",
+            "module_build_id": 2,
+            "module_build_state": 0,
+        }
+        modules.init.delay(**kwargs)
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-2"
+
+    def test_route_modules_wait_task(self, send_task_message):
+        modules.wait.delay("fakemsg", 3, 1)
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-0"
+
+    def test_route_modules_done_task(self, send_task_message):
+        modules.done.delay("fakemsg", 22, 3)
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-1"
+
+    def test_route_modules_failed_task(self, send_task_message):
+        modules.failed.delay("fakemsg", 23, 4)
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-2"
+
+    def test_route_components_build_task_finalize_task(self, send_task_message):
+        scheduler_init_data()
+        components.build_task_finalize.delay(
+            "fakemsg", 123, 90276228, 1, "perl-Tangerine", "0.23", "1.module+f28+2+814cfa39")
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-2"
+
+    def test_route_components_build_task_finalize_task_without_a_module(self, send_task_message):
+        scheduler_init_data()
+        components.build_task_finalize.delay(
+            "fakemsg", 123, 123456, 1, "hostname", "0.1", "1.module+f28+2+814cfa39")
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-default"
+
+    def test_route_repos_done_task(self, send_task_message):
+        scheduler_init_data()
+        repos.done.delay("fakemsg", "module-testmodule-master-20170109091357-7c29193d-build")
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-2"
+
+    def test_route_repos_done_task_without_a_module(self, send_task_message):
+        scheduler_init_data()
+        repos.done.delay("fakemsg", "no-module-build-exist")
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-default"
+
+    def test_route_tags_tagged_task(self, send_task_message):
+        scheduler_init_data()
+        tags.tagged.delay(
+            "fakemsg", "module-testmodule-master-20170109091357-7c29193d-build",
+            "perl-Tangerine", "perl-Tangerine-0.23-1.module+f28+2+814cfa39")
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-2"
+
+    @mock.patch("koji.ClientSession")
+    def test_route_greenwave_decision_update_task(self, kojisession, send_task_message):
+        kojisession.return_value.getBuild.return_value = {
+            "extra": {"typeinfo": {"module": {"module_build_service_id": 1}}}
+        }
+        scheduler_init_data()
+        greenwave.decision_update.delay(
+            "fakemsg",
+            decision_context="test_dec_context",
+            subject_identifier="module-testmodule-master-20170109091357-7c29193d-build",
+            policies_satisfied=False
+        )
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-1"
+
+    def test_route_fail_lost_builds_task(self, send_task_message):
+        fail_lost_builds.delay()
+        queue = send_task_message.call_args[1].get("queue")
+        qname = queue.__dict__.get("name")
+        assert qname == "mbs-default"