Support for v6 address for a VM

1) Schema changes to add instance-ip-family, this is needed as the InstanceIp object is pre-created by the vnc api client code, coverts the object to json and sends the post request. The instance-ip-family is a trigger so the vnc api server code could allocate an instance-ip object per family.
2) neutron plugin changes to pre-create InstanceIp object for each family
3) glue-logic in vnc_addr_mgmt.py and vnc_cfg_types.py