LU-3009 tests: failure on conf-sanity test_17

LU-3009 tests: failure on conf-sanity test_17

Test 17 is failing due to the failure of the debugfs command
in this subtest.  The command fails because MGSDEV is undefined.
Using an alternative that is available in the current test framework.

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I93407dd3c349d91a2ea4690b70dd67c68d65582c
Reviewed-on: http://review.whamcloud.com/5816
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>