Consistent templating across metrics

[acordonez]

Different metrics sometimes use different keywords for the same value in parameter files (e.g. "test_data_set" for mean_climate versus "modnames" mjo). It would be helpful for users and new developers if the same keyword was used for the same test value across the PMP.

Common arguments can be added to https://github.com/PCMDI/pcmdi_metrics/blob/master/share/setup_default_args.py and imported via the PMP Parser.

[gleckler1]

@acordonez This is really great!