Cruise/logger config file validation

[davidpablocohn]

Would be nice to have an easy-to-invoke method/routine to validate the JSON files for logger configs and cruise configs (e.g. the ones in test/configs). Want to use it

1. on command line when logger/listener/listen.py --config_file is used to give a sensible error message.

2. in the GUI when user loads a config file via the load_config dialog. Should display an error in the GUI explaining what's wrong with the file prior to loading it into the DB and creating all its objects.

A logger config should

1. be parseable as (extended) JSON by logger/utils/read_json
2. contain at least a reader, writer and optionally one or more transforms

A cruise config should

1. be parseable
2. contain a cruise id (optionally start and end)
3. at least one mode containing at least one logger
4. a default mode declaration