Some services or users may have obtained an access token, so it would be
possible to authenticate using this token directly (for example a
service where the user has already logged in). This new class makes
possible to use an access token to authenticate directly with Keystone,
exchanging it for a Keystone token.
Closes-bug: 1583780
Change-Id: I5a31270194a3d1aa48de709dba49afde460731e2