Implement handle_update for FloatingIPAssociation resource

1. modify the property "port_id" to be required.
2. Implement handle_update for FloatingIPAssociation resource, allows to
update FloatingIpAssociation with new port_id(and fixedIp if port has
multipleIps) and new floatingip_id.

Change-Id: Ie40669bee849d91ed93446dcf7f066fe6a5b2961
Implements: blueprint handle-update-for-floatingip
Merge "Implement handle_update for FloatingIPAssociation resource"