With bp stevedore, keystone will load backend drivers using
stevedore entrypoints. Using the qualified class name is
deprecated.
Since stevedore is going to validate that the entrypoint is
found, there's no need to list the valid backends, so backend
validation was removed. This change will cause the server to fail
to start if the backends are misconfigured rather than using the
default one.
The names of the stevedore endpoints are "sql", "ldap", etc.,
rather than the qualified class name, so the way that these
are specified in KEYSTONE_IDENTITY_BACKEND, etc., is the same as
the stevedore entrypoint and there's no need to translate.
Change-Id: I81e4e3a6c97b0057610e6b256aff5df4da884e33