LU-3624 iokit: fix mds-survey for multiple MDT

The mds-survey script has several issues when called with multiple
MDT targets. This patch fixes the issues.

- make echo_client devices attached to different MDTs work in
  different directories
- correctly compute global statistics from each MDT stats
- fix mds-survey output so that number of field displayed is
  constant. This allows parsing the output for future plotting.
- fix lctl test_xxx commands to correctly handle specified
  parent directory

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: I3ef0b82b59124f2246db535b074be94c7235936a
Reviewed-on: http://review.whamcloud.com/7558


Tested-by: Maloo <whamcloud.maloo@gmail.com>
Tested-by: Hudson
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>