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
Added field parameter identifying the custom validation

In the template file a field with custom constrain should
return in the parameter arguments a custom constrain value.

Change-Id: I7df56a840eabb2f8093ea82eb822a764e9267b40
Closes-bug: #1313835
Merge "Added field parameter identifying the custom validation"
Merge "Implement handle_update for FloatingIPAssociation resource"