follow atmos approach for config file loading

[juliasloan25]

ClimaAtmos and ClimaCoupler currently have slightly different approaches to setting up the config. Atmos reuses an existing config file if it's defined, or creates a new one if not. Coupler checks if we're running interactively and uses a default in that case, otherwise requires the user to specify a config file and job id. It might be nice to update the coupler approach to more closely align with that of atmos (see atmos code here).

Other things to think about:

• rename interactive_debug.yml -> default_slabplanet.yml ? and create a default_amip.yml? might be useful for those just starting
• extract config and parsed_args setup to a different file/function to increase clarity of driver code