Add tests for config file loader util

[JakeDawkins]

Goals

• Test the loadConfig functionality
• Document/reorganize the logic
• Bugfixes where discovered by tests

What I did

• Fixed bug where a user specifies loading of a config with a flag, but if that file isn't found, it loads an apollo.config.js.
  • Ex: apollo client:check --config=config/apollo.js would also load ./apollo.config.js if it couldn't find the specified config. I think if a user is manually specifying a config, it should only load what they specified and throw an error if not found.
  • This was preventing testing errors, since leaving off a config would just load this repo's config (which wouldn't work)
• Removed the getSearchPlaces function. It was extraneous, and misleading. While search places is the term that dotenv uses, it is less descriptive than defaultFileNames, since that what the search places are–filenames.
• Added try/catch around the cosmiconfig's loader, since it can throw on malformed files with an unhelpful error.
• Handled case where .env is not a File, but a folder or something else. Resolves apollo service:push fails because it makes assumptions about the environment #1132
• Split up project type and service name setting, since those being intertwined added unnecessary complexity
• commented :allthethings:

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.

[trevor-scheer]

Ship it!