1. Now, the vitrage use [service_credentials] to get credentials[0],
remove the unused [keystone_authtoken]
2. Move memcache stuff into service_credentials section

[0]: https://github.com/openstack/vitrage/blob/5c4bd36e8090ecbfca48980bc47c89e4808230b1/vitrage/opts.py#L56

Change-Id: Id22e59ff572a2fef08906bf6cc48578981d2efd6