Add metadata proxy L3 agent driver

To-Do:
* vArmourL3NATAgent (Before this patch) overrides _spawn/destroy_metadata_proxy
  with empty stubs. This behavior should be maintained so that the metadata proxy
  is not created when using vArmourL3NATAgent. It also overrides _router_added
  and _router_removed, causing L3 agent driver notifications to not send out.

Partially-implements: blueprint restructure-l3-agent
Change-Id: I36900b02bff34269f789956aa324379ff51eb81b
Merge "Add metadata proxy L3 agent driver"