Add mpi_runner_type submit option to executor

[shuds13]

The supplied MPI runner will be used for this submit only.

This is useful in combination with the environment script option, when a different MPI runner may be needed for a particular submit.

The option can be given as a string specifying the MPI runner (e.g. "mpich", "openmpi", "srun", "jsrun", "aprun"), or as a dictionary with the same options as custom_info when setting up the executor e.g : {"mpi_runner": "openmpi", "runner_name": "myrunner"}

Provides support for optimas-org/optimas#108

• Add functionality test
• Discuss/agree name of argument (currently mpi_runner_type). Could be mpi_runner, env_mpi, mpi_config etc...
• Check if this branch MPIExecutor works standalone @jlnav

Another change is that when using an environment script, the generated script is logged to ensemble.log rather than the mpirun line itself (which is in the script). This could be either way - any feedback welcome.

Ideally, the mpi_runner_type option would match custom_info in executor init function in name and accepted input.

[jlnav]

Tentative consensus for "mpi_runner"

[jlnav]

second tentative consensus for "mpi_runner_type" ?

[shuds13]

Think I'm going to keep mpi_runner_type