Add `dce auth`; Refactor Config file mgmt; Add integration tests

[eschwartz]

Stuff going on here

• Add dce auth command
  • pops you out to a browser at the /auth endpoint that @joshmarsh is working on
  • see #33
• Remove dce.yml configs we aren't using
• Configure a AWS creds provider that will load creds from the auth token, but fallback to env vars or the ~/.aws/credentials file
• Run dce auth before any command, if we don't have any valid creds in our chain
• Refactor how to load and save our config YAML, so it's easier to work with from inside multiple commands
• Remove viper dependency (wasn't really being used)
• Some other cleanup/refactor around init logic, to get this all wired up properly
  • I'll try to point these out in the code
• New setup for integration tests
  • These test call our cobra CLI programmatically. For some reasons:
  • Lets us use debugger to run through tests
  • Don't need to rebuild for every test run
  • Can still use mocks at boundaries (eg. mock out web browser call)
  • I'm hoping this new testing pattern makes it easier to write tests for CLI commands going forward.