Send quantum_settings:L2:provider as 'nsx'

* Remove alternation of quantum_settings:server:core_plugin
* Send quantum_settings:L2:provider = nsx instead
* Fix Nailgun integration tests

Change-Id: If581b39c0e82f07a0e1a0f8ee9db7da7a6151fe0
Closes-bug: #1358255
Merge "Send quantum_settings:L2:provider as 'nsx'"