Fix unit tests error introduced by _get_session

1. What is the problem
In neutron.plugins.ml2.drivers.helpers._get_session, the passed-in
argument is first checked if it's a neutron.context.Context object,
the argument will be directly returned if it's not a neutron Context
object. In our test cases, the FakeNeutronContext does not inherit
neutron Context object, so the session inside FakeNeutronContext is
not returned.

2. What is the solution to the problem
Make FakeNeutronContext inherit neutron Context object. A bump_revision
method is also added to DotDict object since Neutron starts to call
this method.

3. What the features need to be implemented to the Tricircle
No new features

Change-Id: Ib17eb96418088217e991e52501209e0fc1227213

tricircle/tests/unit/network/test_central_plugin.py

@@ -34,6 +34,7 @@ from neutron_lib.plugins import directory
 
 import neutron.api.v2.attributes as neutron_attributes
 import neutron.conf.common as q_config
+import neutron.context as q_context
 
 from neutron.db import _utils
 from neutron.db import db_base_plugin_common
@@ -253,6 +254,9 @@ class DotDict(dict):
     def __copy__(self):
         return DotDict(self)
 
+    def bump_revision(self):
+        pass
+
 
 class DotList(list):
     def all(self):
@@ -627,13 +631,12 @@ class FakeClient(object):
         pass
 
 
-class FakeNeutronContext(object):
+class FakeNeutronContext(q_context.Context):
     def __init__(self):
         self._session = None
         self.is_admin = True
         self.is_advsvc = False
         self.tenant_id = TEST_TENANT_ID
-        self.project_id = TEST_TENANT_ID
 
     @property
     def session(self):