LU-1187 mdt: Add pre_cleanup phase in MDT stack cleanup

LU-1187 mdt: Add pre_cleanup phase in MDT stack cleanup

Add pre_cleanup phase in MDT stack cleanup, so MD OSP can be
disconnected before cleanup MDT stack.

Signed-off-by: wang di <di.wang@whamcloud.com>
Change-Id: Id1a56c393c22b4fc526c81c05b11bfb656a80559
Reviewed-on: http://review.whamcloud.com/4929
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>