Move process_ha_router_added/removed from HA agent to router

Move process_ha_router_added/removed from HA agent to router

* Move process_ha_router_added/removed from ha.py to
  ha_router.py, rename them initialize and terminate
* Remove _process_ha_router (Spawns/disables keepalived) from
  process_router (Called when adding/updating and deleting
  a router), move its content to process_router for add/update
  and terminate for delete
* Rename ha_router.spawn_keepalived to enable_keepalived
  (Consistent with disable_keepalived and process_manager
  semantics)

Partially-Implements: bp/restructure-l3-agent
Change-Id: I1f21acdae2ae1faa2c78affaa3f1ce9056487104