LU-13941 test: Restrict create_count to valid range

LU-13941 test: Restrict create_count to valid range

create_count range is restricted to:
  [OST_MIN_PRECREATE=32, OST_MAX_PRECREATE=20000]

Enforce this restriction rather that halt with an error.

Test-Parameters: trivial testlist=parallel-scale
HPE-bug-id: LUS-5960
Fixes: 21501dedf64e ("LU-9780 tests: Testing Round-Robin allocation")
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I125f2121a57d16a9031ed21938ce2f0b99b9dea8
Reviewed-on: https://review.whamcloud.com/39798
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>