Implement NovaFloatingIpAssociation res updatable

Implement handle_update for NovaFloatingIPAssociation
resource, allows to update NovaFloatingIpAssociation with
new server_id and new floatingip_id.

Change-Id: I75d361e2f0381f50ab83b54d752ffc080fe337fa
Implements: blueprint handle-update-for-floatingip
Merge "Implement NovaFloatingIpAssociation res updatable"