Correct service_credentials section

Correct service_credentials section

The service_credential section variables were renamed at some point
before Mitaka. They are broken in Mitaka and Newton, and possibly
earlier. Additionally some critical variables for dealing with domains
and keystone v3 are missing. This corrects those issues.

A later commit should deprecate some of the naming for things
that have totally different names, like the endpoint_type and
tenant_name, but we need to cleanly (and minimally) backport this to
Mitaka.

Change-Id: I8de5e42102fed2de6bea7bbdd788e86a88f0354c
Closes-Bug: #1581691