LU-718 mdt: rename mdt threads parameters to mds

LU-718 mdt: rename mdt threads parameters to mds

rename mdt threads parameters to mds, so the following
mds layer change does not need to deal with the compatiblity
issue.

Signed-off-by: Wang Di <di.wang@whamcloud.com>
Change-Id: I3cb095812488b9459e4a3e878757d40410ac4191
Reviewed-on: http://review.whamcloud.com/3377
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>