use net ip availability api def from neutron-lib
use net ip availability api def from neutron-lib

Commit I44451faa51b47e14ea17c60c035f3f497f00d7f6 add the network IP
availability extension's API definition to neutron-lib. This patch
consumes it by removing the extensions code that now lives in lib
as well as having the extension class inherit from the base
APIExtensionDescriptor class.

NeutronLibImpact

Change-Id: Iddfaaf01db05330109a9a3c10d05e02b6a2b9d09

eayunstack/neutron,noironetworks/neutron,eayunstack/neutron,huntxu/neutron,mahak/neutron,openstack/neutron,mahak/neutron,huntxu/neutron,openstack/neutron,noironetworks/neutron,openstack/neutron,mahak/neutron