LU-13272 tests: customise dom-performance

LU-13272 tests: customise dom-performance

dom-performance compares the performance benefits
between the normal file and DoM file.
However it is also interesting to demostrate that if
the file grows beyond the DoM size the performance
for it is not worse compared to the normal file.

Patch allows to set file size beyond the DoM size.

Test-Parameters: trivial mdssizegb=20 \
testlist=dom-performance envdefinitions=ONLY=IOR
Cray-bug-id: LUS-7359
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: I2412b4f1e1d0a591c47a9ceadc37f730c4deedbe
Reviewed-on: https://review.whamcloud.com/37650
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>