Local builds: avoid 'Hub not initialized. Queueing on the side' warnings

We don't need messages at all for local builds, so use a separate "drop"
backend and reserve the "in_memory" backend for tests, where we sometimes
want to inspect the messages. This avoids a warning for each published
message.

module_build_service/common/config.py

@@ -75,7 +75,7 @@ class ProdConfiguration(BaseConfiguration):
 class LocalBuildConfiguration(BaseConfiguration):
     CACHE_DIR = "~/modulebuild/cache"
     LOG_LEVEL = "debug"
-    MESSAGING = "in_memory"
+    MESSAGING = "drop"
 
     ALLOW_CUSTOM_SCMURLS = True
     RESOLVER = "mbs"

module_build_service/common/messaging.py

@@ -95,6 +95,10 @@ def _in_memory_publish(topic, msg, conf, service):
         _initial_messages.append(wrapped_msg)
 
 
+def _drop_publish(topic, msg, conf, service):
+    """ Drop the message on the floor (used for local builds). """
+
+
 known_fedmsg_services = ["buildsys", "mbs"]
 
 
@@ -110,6 +114,12 @@ _in_memory_backend = {
     "services": [],
     "topic_suffix": ".",
 }
+_drop_backend = {
+    "publish": _drop_publish,
+    "parser": FedmsgMessageParser(known_fedmsg_services),  # re-used.  :)
+    "services": [],
+    "topic_suffix": ".",
+}
 
 
 _messaging_backends = {}

setup.py

@@ -47,6 +47,7 @@ setup(
         "mbs.messaging_backends": [
             "fedmsg = module_build_service.common.messaging:_fedmsg_backend",
             "in_memory = module_build_service.common.messaging:_in_memory_backend",
+            "drop = module_build_service.common.messaging:_drop_backend",
             # 'custom = your_organization:_custom_backend',
         ],
         "mbs.builder_backends": [