LU-11586 tests: test_10() fix for FILESET set

Patch fixes sanity-pfl test_10() to set root layout
properly: subdirectory could be mounted on $MOUNT if FILESET
is set, test now mounts root on $MOUNT2 for this case and
sets the root layout via $MOUNT2.

Test-Parameters: trivial testlist=sanity-pfl envdefinitions=ONLY=10
Signed-off-by: Elena Gryaznova <c17455@cray.com>
Cray-bug-id: 6557
Reviewed-by: Andriy Skulysh <c17819@cray.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Change-Id: If94474d92686a37518c1dd51793e742967fdfbc1
Reviewed-on: https://review.whamcloud.com/33514
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>