Standardize CIME submodule conventions

[jasonb5]

The CIME submodule can be placed anywhere as long as there's a config_files.xml in the CIME repo. This is a major coupling between the model and CIME, which without CIME would not be able to find the models config.

The proposal is the standardize the CIME config directory name and placement within the model repo. With this we can remove this couple as CIME whenever used a submodule will be able to detect the models $SRCROOT.

Related #3923

[rljacob]

Discussion in telecon: currently CIME looks in CIME/data/config/$CIME_MODEL/config_files.xml . For e3sm, config_files.xml is just a pointer to a bigger config_files.xml in the E3SM source, E3SM/cime_config/config_files.xml. Proposal is to standardize that first the cime repo will be installed in the top level of the parent model AND that there will also be a "cime_config" directory at the same level where it can find config_files. Will remove the need to have a CIME/data/config/$CIME_MODEL directory.

[billsacks]

Connection to #3923 (without having totally gotten my head back into that issue): I think we could replace the current heuristic of checking for a share directory with a new, more codified heuristic that looks for config_files.xml in the parent directory to determine if we're within a model as opposed to standalone cime.

[jasonb5]

The order for determining SRCROOT will be:

• environment variable
• user config under ~/.cime
• flag --srcroot
• step up one directory from CIMEROOT
• OPTIONAL check CIMEROOT, may be used if CIME add a deterministic stub-model for testing

Validation will check for cime_config e.g. $SRCROOT/cime_config. If the validation fails it's assumed CIME is operating in standalone mode (valid for certain uses e.g. unittesting).

The XML system uses https://github.com/ESMCI/cime/blob/025f09aba0b72177df7e22705f0eadbe351b5add/CIME/data/config/e3sm/config_files.xml to find the models config_files.xml. This will be removed and the XML system will now use the validated SRCROOT to walk find all files required to initialize the system. Anything missing will cause early failure.