Add RPC command and delete if last FIP on Agent

Add RPC command and delete if last FIP on Agent

Today FloatingIP Agent gateway port is deleted and
re-created for DVR based routers based on floatingip
association and disassociation with VMs on compute
nodes by the plugin.

This introduces lot more strain on the plugin to
create and delete these ports when VMs come up and
get deleted that are associated with FloatingIps.

This patch will introduce an RPC call for the agent
to initiate a agent gateway port delete.

Also the agent will look for the last floatingip that
it manages, and if condition satisfies, the agent will
request the server to remove the FloatingIP Agent
Gateway port.

Change-Id: I47694b2ee60c363e2fe59ad5f7d168252da08a45
Related-Bug: #1468007
Related-Bug: #1408855
Related-Bug: #1450982