Expose public_interface_tag support.

Proper VLAN support requires adding the IP address to a new device,
rather than br-ex/br-ctlplane. This is added in the
tripleo-image-elements change https://review.openstack.org/103449
(I3f77f72ac623792e844dbb4d501b6ab269141f8e) and here we just expose
it with appropriate glue to get the IP address from Neutron.

With this we can now describe a VLAN public interface scenario
to the undercloud and overcloud control planes.

Change-Id: I4d2194fc813aebb0708d6fddf4f05bae5f091fd8
Merge "Expose public_interface_tag support."