LU-8655 tests: customize run_mdtest()

LU-8655 tests: customize run_mdtest()

Sometimes it is required to run mdtest with parameters
missing in run_mdtest() cmd.
Now these parameters can be specified by mdtest_custom_params.

Test-Parameters: trivial envdefinitions=ONLY=mdtestssf testlist=parallel-scale
Test-Parameters: trivial envdefinitions=ONLY=mdtestfpp testlist=parallel-scale
Seagate-bug-id: MRP-3376
Signed-off-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@seagate.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@seagate.com>
Change-Id: If07f07ebf11516195843e497c5f97bbdadeb531b
Reviewed-on: http://review.whamcloud.com/22850
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>