The admin interface for endpoints never had any real use, the
functionality was the same as for the public or internal endpoints,
except for Keystone. Even for Keystone with API v3 it would no longer
really be needed, but it is still being required by some libraries that
cannot be changed in order to stay backwards compatible.

Signed-off-by: Dr. Jens Harbott <harbott@osism.tech>
Change-Id: Icf3bf08deab2c445361f0a0124d87ad8b0e4e9d9