LU-14274 tests: enhance racer to set extra layout

LU-14274 tests: enhance racer to set extra layout

Patch adds an ability to set:
  - a generic "RACER_EXTRA_LAYOUT" contained any kind of
    layout in addition to layouts defined for
    RACER_ENABLE_*;
  - an initial racer RACER_PROGS commands list.
    The additional commands specified by RACER_EXTRA,
    RACER_ENABLE_REMOTE_DIRS, RACER_ENABLE_STRIPED_DIRS
    and RACER_ENABLE_MIGRATION are not ignored, i.e. the
    following parameters are to be set to run file_create only:
        RACER_ENABLE_REMOTE_DIRS=false
        RACER_ENABLE_STRIPED_DIRS=false
        RACER_ENABLE_MIGRATION=false.
Patch fixes NUM_THREADS and MAX_FILES to be passed correctly.

Test-Parameters: testlist=racer
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-9142
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: I9994dfdb0555a3acd75daa4cfd27a0cb62074e36
Reviewed-on: https://review.whamcloud.com/41077
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>