Introduce status for floating IPs

Introduce status for floating IPs

Add a new 'status' attribute to the floating IP resource.
Extend the plugin RPC interface for allowing status updates from agents,
and implement support for operational status in the L3 agent.

The default behaviour for all the plugins different from
neutron.services.l3_router.l3_router_plugin is to set the status of
a floating IP to ACTIVE upon creation.

Implements blueprint fip-op-status
Change-Id: Ib0176fbcde95b0db5dfdabd4fc297d1d29c0d604