BVS-744: Add hook for diassociate_floatingip to plugin

We need to send the change in the state (disassocation of the floating ip)
to the controller. Adding this hook.

We send an update on the external network
Merge branch 'grizzly/stable' of http://github.com/bigswitch/quantum into grizzly/stable

Conflicts:
	quantum/plugins/bigswitch/plugin.py