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
Merge "Ensure OVS plugin is loaded in OVS plugin test"