Fixing external interface adding to OVS external/data bridge

Previously this recipe could add external (public) interface named
<ext_bridge_iface> only together with external bridge <ext_bridge>
creation. If for some reason this port will be deleted from bridge chef
won't never add it again until external bridge will be removed. Now this
case is handled properly. If desired external interface is already
plugged into another OVS bridge, recipe generates exception with
detailed information and interrupts chef-client execution.