Ensure OVS plugin is loaded in OVS plugin test

Ensure OVS plugin is loaded in OVS plugin test

In TestOpenvswitchSGServerRpcCallBack and NetworkBindingsTest
in OVS plugin tests, OVS plugin was not loaded properly.

* For NetworkBindingsTest, it can be fixed by changing setUp()
  to take 'plugin' arguments in the base test classes
  SecurityGroupDBTestCase and SGServerRpcCallBackMixinTestCase.
  This fixes bug 1242510 in ML2 unit tests.

* For NetworkBindingsTest, it can be fixed by passing the plugin
  class to super.setUp(). The test itself needs to be updated
  because network binding is added when a network is created.

* NetworkBindingsTest in Linux Bridge plugin has the same issue,
  so it is also fixed in this patch.

Closes-Bug: #1230083
Closes-Bug: #1242510
Change-Id: I914876225480585d822748c188e9b69d1adf46f3