LU-11522 tests: sanity-sec/27 unable to remount client

LU-11522 tests: sanity-sec/27 unable to remount client

test_27() defect: test expects that lustre root is mounted on
$MOUNT which is not true if t-f runs witn FILESET set.

Patch fixes the test to have lustre root mounted before test
starts and restores the FILESET mount in the test cleanup.

Test-Parameters: trivial testlist=sanity-sec
Signed-off-by: Elena Gryaznova <c17455@cray.com>
Cray-bug-id: LUS-6560
Change-Id: I00cf50e00ed96d55552f010b5766a8bdab75a200
Reviewed-on: https://review.whamcloud.com/33389
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: Sergey Cheremencev <c17829@cray.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>