LU-11078 tests: customise run_ior()

LU-11078 tests: customise run_ior()

Sometimes it is required to run ior with parameters
missing in run_ior() cmd, custom mpi threads per
cient and custom striping.

Now these parameters can be specified by:
  ior_custom_params
  mpi_ior_custom_threads
  ior_stripe_params

Test-Parameters:trivial testlist=parallel-scale
Signed-off-by: Elena Gryaznova <c17455@cray.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Change-Id: I785d7aece107b2b53d0c422fbaec5c469b5bbc97
Reviewed-on: https://review.whamcloud.com/32688
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>