LU-12390 tests: customise mdsrate tests

LU-12390 tests: customise mdsrate tests

Patch adds the following customizations:
  set custom layout for filesystem root
  directory by specifying fs_STRIPEPARAMS
  and
  set the custom layout for mdsrate test's
  working directory BASEDIR by specifying
  mdsrate_STRIPEPARAMS.

Test-Parameters: trivial testlist=performance-sanity
Signed-off-by: Elena Gryaznova <c17455@cray.com>
Cray-bug-id: LUS-7249
Reviewed-by: Alexander Boyko <c17825@cray.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Change-Id: I28524c203f86e7e94a4db47a8d8c4b3518883600
Reviewed-on: https://review.whamcloud.com/35068
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>