Don't replace OS::Neutron::Port on update

Due to an ununsual interface to OS::Neutron::Port resources,
it's necessary to specify replacement_policy: AUTO, or the
resource is unconditionally replaced on every stack update.

I've started discussion re possibly changing the default in
Heat, but right now, we need this or we have the bad outcome
of replacing all (!) compute and controller nodes on every
stack-update, even if the templates are unmodified.

Passing the AUTO value should be safe regardless of any
potential change of default value in Heat.

Change-Id: I6dd02ae17407f8f4c81ae418e5027f4f38ae4e9b
Closes-Bug: #1383709