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

This change is congruent with I6dd02ae17407f8f4c81ae418e5027f4f38ae4e9b
but applies to undercloud configs rather than overcloud configs.

I've listed this as closing 138709 even though that bug didn't talk
about the undercloud as this seems like it's another instance of the
same issue seen there.

Change-Id: I3ee80043bb455460991e78525fa4310934df4697
Closes-Bug: #1383709
Merge "Don't replace OS::Neutron::Port on update of undercloud"