Config loading refactor

This PR relates to issue: https://github.com/jembi/openhim-core-js/issues/177
Add in the functionality to load a custom config file.
Moved the Mongo and logger settings into default.json config
custom config file can be used by the follow start up variable ( --path=path/to/config.json )
Super minor note, but it might be better called `environmentOverride` since development is only the default. The same with the comment.