LU-12488 tests: Fix sanityn 93 for DNE configs

LU-12488 tests: Fix sanityn 93 for DNE configs

sanityn test 93 only uses MDT0, but it is getting the parameter
qos_threshold_rr for all MDTs.  This confused the test when
trying to reset the parameter value.

Just limit everything to MDT0. Also modernize lctl vs
$LCTL usage and replace hardwired file system name with
$FSNAME.

Test-Parameters: trivial
Test-Parameters: testlist=sanityn
Test-Parameters: fstype=ldiskfs clientcount=2 mdscount=2 mdtcount=4 osscount=1 ostcount=8 testlist=sanityn
Test-Parameters: fstype=zfs clientcount=2 mdscount=2 mdtcount=4 osscount=1 ostcount=8 testlist=sanityn

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Iff9cca4ce3d2e5ad4e499bba0369189bea21448a
Reviewed-on: https://review.whamcloud.com/35366
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>