LU-718 mds: new MDS layer.

Pull out ptlrpc service from MDT and create new MDS layer.

Change-Id: Ib8a5d22c1f85d086f6ed37e4c46c7aa0e3c4b03a
Signed-off-by: Wang Di <di.wang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4354
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>