Add option to put seed public interface on OVS bridge

Add an option(-p) which names a OVS bridge on which to place the
seed public interface. This will be needed by the CI which expects
this interface to be on a OVS bridge rather then the libvirt default
network.

Change-Id: Ia006bbd78a1abf6ef5b0204e8b883bffdb9cf2a5