Allow update of ext gateway IP's w/out port delete

Allow update of ext gateway IP's w/out port delete

(Patch set #5 for the multiple-ipv6-prefixes blueprint)

Updating an external gateway port currently triggers a port-delete
followed by a port-create. In the multi-prefix case, if a second
subnet is added to an external gateway port, the port will be
deleted, freeing the original IP allocation, and then the port will
be recreated with new IP allocations from the two subnets. This is
undesirable as the port can't keep the same IP address from the
original subnet.

This patch modifies the behavior so that a fixed-ip change on an
external gateway port will cause a port-update instead of a
delete/create. If the gateway port network id has changed, however,
the port will be deleted and recreated as before.

Change-Id: I5b19d3b167668ce5c04e7ce8adc63249a4501d0e
Partially-implements: blueprint multiple-ipv6-prefixes