Add floating IP test to ensure backwards compat
Add floating IP test to ensure backwards compat

This adds an API test to ensure that we maintain the behavior
of disassociating a floating IP if it receives an update with
an empty body.

Change-Id: I9efd941b621e15691af95cd88f391aa564e4b9bd
Related-Bug: #1607746

sebrandon1/neutron,mahak/neutron,igor-toga/local-snat,mahak/neutron,noironetworks/neutron,cloudbase/neutron,openstack/neutron,eayunstack/neutron,openstack/neutron,igor-toga/local-snat,mahak/neutron,noironetworks/neutron,eayunstack/neutron,huntxu/neutron,sebrandon1/neutron,openstack/neutron,cloudbase/neutron,huntxu/neutron