LU-13280 tests: customize run_mdtest for several mount points

LU-13280 tests: customize run_mdtest for several mount points

Patch adds the possibility to run mdtest on $mdtest_Nmntp
number of Lustre mount points.

Test-Parameters: trivial testlist=parallel-scale \
envdefinitions="mdtest_Nmntp=3" \
envdefinitions=ONLY=mdtestfpp envdefinitions=ONLY=mdtestssf
Cray-bug-id: LUS-7686
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Change-Id: I9bc68d4f296970290cf460bf25cc26537bcc3281
Reviewed-on: https://review.whamcloud.com/37665
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>