rose mpi-launch: relax requirement for >=1 args.

Should allow use of a command file for some mpi launchers.
rose mpi-launch: tidy up usage.
rose mpi-launch -f FILE: new option.

Support launching with a command file.
rose mpi-launch: add command launch tests.
rose mpi-launch: allow default command file.

If "rose-mpi-launch.rc" exists in $PWD, it will be assumed as the
command file to submit to the launcher program.

More tests are also added in this commit.