Move the metadata NAT rule to nova-baremetal.

It was part of the openvswitch agent rules, which is entirely
inappropriate for overclouds.

Change-Id: I43faa086f5625434f9abdea68a3bb88413688d4c
Set a bridge id for external bridges.

Unlike br-int we were not setting a bridge-id. I'm not sure if it's
needed, but it is consistent to add one, and will allow consolidating
other code which currently differs only by setting such an id.

Change-Id: I0e5ada816948cc7fd3a5fe7b5b92b7bb89ebaf5a