Correct vswitch external interface detection

When the vswitch port is added to a bridge with `keep_ip => true` the ip address migrates to the bridge device. On the next puppet run, the `device_for_network()` will then detect the bridge instead of the original interface.

To avoid this, this commit decides that if the ip address is on the bridge device, then the `vs_port` has already been created. One caveat would be that if the external bridge name was ever changed, there would not be a way to re-link the interface from one bridge to another.

An alternative solution would be to remove `keep_ip => true`, but I don't know the implications of doing this.