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.
This uses an in-memory table of credentials keyed on hostname. This is
the simplest possible credentials source that can actually return
credentials, and is suitable for representing statically-configured
credentials from configuration.