LU-4830 tests: only deactivate MDTs of Lustre FSNAME

LU-4830 tests: only deactivate MDTs of Lustre FSNAME

Problem has been discovered using a very specific copytool Lustre
root/archive usage. All MDCs, including archive one, were
deactivated.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I359e001e001b931e0a8ed773e0d476ffc3ea44db
Reviewed-on: http://review.whamcloud.com/9843
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>