LU-676 tests: machinefile option for mpirun via a variable

Not all MPI implementations pass the host file to mpirun via the same
option. Common options are -machinefile and -hostfile.

This problem can be resolved by using a variable MACHINEFILE_OPTION
instead. A default value is assigned if the variable not defined.

Signed-off-by: Jay J Lan <jay.j.lan@nasa.gov>
Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I3362a6e62a27318cff733aea2f99b1356b3ff02e
Reviewed-on: http://review.whamcloud.com/3873
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>