Maintainability improvements and enhancements

[bodom0015]

Capturing feedback from @manning-ncsa about the preliminary prototype:

Other items I noticed while reviewing the latest code version that I'll list here for reference:

• Instead of modifying the existing job endpoint handler functions, duplicates with specific "CLEAN" names were created. This seems unnecessary and confusing.
• In CLEANSubmitJobHandler "post" function, the BaseHandler.getarg() function is bypassed for some reason. Also this function semantically makes more sense as a PUT request as in the original.
• Same comment for CLEANStatusJobHandler POST reverting to GET for semantic reasons
• The command parameter is hard-coded in the kubejob.create_job() function instead of being set in the calling request handler function CLEANSubmitJobHandler which makes kubejob.create_job() unnecessarily specific.
• There is a new APPCONFIG variable in main.py whose config info should instead be inserted into config/server.yaml where it can be accessed like all the other application config data, perhaps at YAML node uws.job.resultBaseUrl or similar

Some of this will be helped by using OpenAPI (as mentioned in #1), and will give more flexibility if a POST needs to change to a GET etc