This new functionality will be used as part of implementing the
"terraform login" and "terraform logout" commands.
As of this commit, the storage codepaths are all just stubs. Subsequent
commits will implement these new methods for each of the different
physical credentials sources.
By adding this method you now only have to pass a `*disco.Disco` object around in order to do discovery and use any configured credentials for the discovered hosts.
Of course you can also still pass around both a `*disco.Disco` and a `auth.CredentialsSource` object if there is a need or a reason for that!
Martin Atkins