Add source context to project API return

Currently api/tenant/openstack/project/ returns a list of config
objects but doesn't give you information on where this config
was defined.  Add the source context to the result.

Change-Id: I741d2e935ac6ed23035c40c079b36fd364386577

tests/unit/test_web.py

@@ -720,6 +720,9 @@ class TestWeb(BaseTestWeb):
                 'connection_name': 'gerrit',
                 'name': 'org/project1',
                 'configs': [{
+                    'source_context': {'branch': 'master',
+                                       'path': 'zuul.yaml',
+                                       'project': 'common-config'},
                     'templates': [],
                     'default_branch': 'master',
                     'queue_name': 'integrated',

zuul/model.py

@@ -4479,6 +4479,7 @@ class ProjectConfig(ConfigObject):
 
     def toDict(self):
         d = {}
+        d['source_context'] = self.source_context.toDict()
         d['default_branch'] = self.default_branch
         if self.merge_mode:
             d['merge_mode'] = list(filter(lambda x: x[1] == self.merge_mode,