Make OVS and LinuxBridge trunk drivers' is_loaded() property more robust

If ML2 is not in use, CONF.ml2.mechanism_drivers is not a registered
option and thus loading these drivers leads to a NoSuchOptError error.
This patch ensures that the exception is handled and the loading state
of the OVS and Linuxbridge driver is set to False.

Partial-implements: blueprint vlan-aware-vms

Change-Id: I0a6994b9248e878e314661f235ceadb7ce9bbbff

neutron/services/trunk/drivers/linuxbridge/driver.py

@@ -35,7 +35,10 @@ class LinuxBridgeDriver(base.DriverBase):
 
     @property
     def is_loaded(self):
-        return NAME in cfg.CONF.ml2.mechanism_drivers
+        try:
+            return NAME in cfg.CONF.ml2.mechanism_drivers
+        except cfg.NoSuchOptError:
+            return False
 
     @classmethod
     def create(cls):

neutron/services/trunk/drivers/openvswitch/driver.py

@@ -44,7 +44,10 @@ class OVSDriver(base.DriverBase):
 
     @property
     def is_loaded(self):
-        return NAME in cfg.CONF.ml2.mechanism_drivers
+        try:
+            return NAME in cfg.CONF.ml2.mechanism_drivers
+        except cfg.NoSuchOptError:
+            return False
 
     @classmethod
     def create(cls):

neutron/tests/unit/services/trunk/drivers/linuxbridge/test_driver.py

@@ -27,6 +27,8 @@ class LinuxBridgeDriverTestCase(base.BaseTestCase):
         cfg.CONF.set_override('mechanism_drivers',
                               ['a', 'b'], group='ml2')
         self.assertFalse(inst.is_loaded)
+        cfg.CONF.set_override('core_plugin', 'my_foo_plugin')
+        self.assertFalse(inst.is_loaded)
 
     def test_driver_properties(self):
         inst = driver.LinuxBridgeDriver.create()

neutron/tests/unit/services/trunk/drivers/openvswitch/test_driver.py

@@ -46,6 +46,11 @@ class OVSDriverTestCase(base.BaseTestCase):
         ovs_driver = driver.OVSDriver.create()
         self.assertTrue(ovs_driver.is_loaded)
 
+    def test_driver_is_not_loaded(self):
+        cfg.CONF.set_override('core_plugin', 'my_foo_plugin')
+        ovs_driver = driver.OVSDriver.create()
+        self.assertFalse(ovs_driver.is_loaded)
+
     @mock.patch('neutron.services.trunk.utils.gen_trunk_br_name')
     def test_vif_details_bridge_name_handler_registration(self,
                                                           mock_gen_br_name):