Generate koji tag from MBS and use informative name

koji now supports tags with max length of 256, we can use
more informative tag name instead of the hash one.

The new format of koji tag name is:

    module-<name>-<stream>-<version>-<context>

However when the generated tag's length > (256 - len('build')), we
fallback to the old way of name in hash format (module-<hash>).

In this change, koji tag is always generated from MBS itself, even
with pdc resolver.

FIXES: #918 #925

tests/test_utils/test_utils.py

@@ -535,6 +535,22 @@ class TestUtils:
             for c in module_build.component_builds:
                 assert c.weight == 1.5
 
+    def test_generate_koji_tag_in_nsvc_format(self):
+        name, stream, version, context = ('testmodule', 'master', '20170816080815', '37c6c57')
+
+        tag = module_build_service.utils.generate_koji_tag(name, stream, version, context)
+
+        assert tag == 'module-testmodule-master-20170816080815-37c6c57'
+
+    def test_generate_koji_tag_in_hash_format(self):
+        name, version, context = ('testmodule', '20170816080815', '37c6c57')
+        stream = 'this-is-a-stream-with-very-looooong-name' + '-blah' * 50
+        nsvc_list = [name, stream, version, context]
+
+        tag = module_build_service.utils.generate_koji_tag(*nsvc_list)
+        expected_tag = 'module-1cf457d452e54dda'
+        assert tag == expected_tag
+
 
 class DummyModuleBuilder(GenericBuilder):
     """