LU-13283 tests: add racer to nonmpi load

LU-13283 tests: add racer to nonmpi load

Patch adds the ability to run racer as one of
nonmpi loads. All racer parameters can be set over
RACERP:
Example:
  RACERP="MDSCOUNT=3 DURATION=600 RACER_ENABLE_STRIPED_DIRS=false"
etc.

ha_start_mpi_loads() is improved to not create
machinefiles if no mpi load set.

Test-Parameters: trivial
Cray-bug-id: LUS-8297
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Change-Id: I974838ce199897284396ef53d1a487d1a1ae1774
Reviewed-on: https://review.whamcloud.com/37671
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>