Cleanup MPLS OVS driver

    Cleanup MPLS OVS dataplane driver
        * apply NOVA hybrid VIF driver hack only for tap interfaces
        * default OVS bridge name in example config
        * minor code simplification
        * doc cleanup

    bagipe-rest-attach: remove hybrid VIF driver hack, allow prefixes
        * do not use a 'qvo' interface name, that was required only for a
          previous version of the MPLS OVS driver
        * support specifiying prefixes for interfaces attachements

    README update