Add linuxbridge plugin support

* Add quantum_plugin_linuxbridge custom type
  Custom type used to configure linuxbridge plugin
  in linuxbridge_conf.ini

* Add quantum::agents::linuxbridge class
  Setups linuxbridge quantum agent.

* Add quantum::plugins::linuxbridge class
  Setups linuxbridge plugin for quantum server.

Change-Id: I864745d6b5acd8203cf32c371fc2967e3566ae6c
Merge "Add linuxbridge plugin support"