Get some unit testing defaults from config_machines

[billsacks]

From @billsacks on August 4, 2015 17:6

With the changes here:

CESM-Development#87

a number of machine-specific settings need to be explicitly specified when running run_tests.py; these should be obtained from config_machines:

• mpilib
• maybe the mpirun command
• maybe the default compiler for a machine

@quantheory suggested this; he and @jedwards4b seem like the ones who would know how to do this.

Copied from original issue: CESM-Development#92

[billsacks]

It looks like mpilib and compiler can pick up defaults now, although some tweaks are needed:

• For mpilib: fix documentation of this argument in run_tests.py to say it's optional, and test to make sure it works

• For compiler: get rid of the default in the parser.add_argument call: parser.add_argument("--compiler", default="gnu",, so that it defaults to None.

Ideally, the mpirun command would still be picked up by default. In addition, it would be nice if --use-openmp could be determined by default, maybe based on some new xml variable that says whether the pFUnit build requires openmp? But for now, maybe the easiest is to default to this being true, since that's the way we typically build pFUnit. Then could have a flag to set it to false, like --no-openmp.