Do not contest locks in PythonGit

The config writer should be
used in a context manager form to ensure locks are shared and
the data is properly written out. In python 2 it looks as if
the locks were not properly handled and multiple config
writers would just lock the same file (and write) blindly.
In python 3, the context manager is required to not raise
an exception when writing the config due to the lock already
being held by another config_writer.

Change-Id: I42a9804638c6065127ce31a1865b017bf969855f

tests/base.py

@@ -1169,9 +1169,9 @@ class ZuulTestCase(BaseTestCase):
         path = os.path.join(self.upstream_root, project)
         repo = git.Repo.init(path)
 
-        repo.config_writer().set_value('user', 'email', 'user@example.com')
-        repo.config_writer().set_value('user', 'name', 'User Name')
-        repo.config_writer().write()
+        with repo.config_writer() as config_writer:
+            config_writer.set_value('user', 'email', 'user@example.com')
+            config_writer.set_value('user', 'name', 'User Name')
 
         fn = os.path.join(path, 'README')
         f = open(fn, 'w')

zuul/configloader.py

@@ -17,7 +17,7 @@ import yaml
 
 import voluptuous as vs
 
-import model
+from zuul import model
 import zuul.manager
 import zuul.manager.dependent
 import zuul.manager.independent