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!
For situations where no credentials are needed but where a working
CredentialsSource is still required, this variable provides a convenient
way to get a fully-functional-but-empty credentials source.
This credentials source wraps another and caches its results in memory
in a map that is keyed on the request hostname.
This should ease the common case of many services operating on the same
hostname by allowing an initial set of credentials to be re-used for
subsequent requests.
Sander van Harmelen