Implement Authorization logic from configuration/command line

[shreyb]

Authorization

1. Use command-line --auth-method or authorization.auth_method to determine the auth method. No value in either of the above defaults to tokens, and indeed, the value given for authorization.auth_method in the configuration file template will be "token" as is in the original proposal.
2. Based on (1), determine whether there is a custom path to the credential using a) command line flags, b) appropriate config file setting. If both (a) and (b) are blank, then use default discovery method to determine credential paths.