Enable RUN-specific APP configurations

[DavidHuber-NOAA]

What new functionality do you need?

Cycled S2SWA experiments should run the S2S app for a gdas RUN and S2SWA for a gfs RUN. However, as is, this requires some kludges to strip the WA when in the gdas cycle. It would be more convenient, robust, and adaptable to be able to specify the APP for each RUN explicitly.

What are the requirements for the new functionality?

XMLs can be generated for any combination of APPs for varying RUNs (with the caveat that not all configurations will be supported).

Acceptance Criteria

Existing experiments' APP configurations do not change, rocoto XMLs generated by the setup scripts are identical to develop for existing CI tests, and all CI tests pass. Perhaps also running a one-time test with an arbitrary configuration (e.g. S2S gdas with ATM gfs).

Suggest a solution (optional)

config.base will have an APP specified by RUN. This will also require new options in setup_expt.py to take different APPs. Lastly, the setup_xml.py classes (at least AppConfig) will need to have logic added to determine the APP based on each RUN rather than using the same APP for each RUN.