Plugin aware extensions should also be reset at each test setup

PluginAwareExtensionManager caches extension according plugin.
At each db plugin test setup, testing plugin and router are newly
created.
However, the extension manager held the extensions for first time
loaded plugin. Because of their cache.

So if a plugin conditional extension exists, it could not be loaded by
testing order.

Fixes bug 1043381.

Change-Id: Ic994aab61c3d67aa9d422594484049f41ff8b321

quantum/tests/unit/test_db_plugin.py

@@ -34,6 +34,7 @@
 from quantum.db import api as db
 from quantum.db import db_base_plugin_v2
 from quantum.db import models_v2
+from quantum.extensions.extensions import PluginAwareExtensionManager
 from quantum.manager import QuantumManager
 from quantum.openstack.common import cfg
 from quantum.openstack.common import timeutils
@@ -63,6 +64,8 @@ def setUp(self, plugin=None):
         db._MAKER = None
         # Make sure at each test a new instance of the plugin is returned
         QuantumManager._instance = None
+        # Make sure at each test according extensions for the plugin is loaded
+        PluginAwareExtensionManager._instance = None
         # Save the attributes map in case the plugin will alter it
         # loading extensions
         # Note(salvatore-orlando): shallow copy is not good enough in