Default to project admin for preprovisioned admin creds

when using pre-provisioned creds, asking for 'admin' creds may
currently return whatever creds with admin role,
including system admin.
That unfortinately does not work for some services, especially
those that still use project id as part of their endpoint
(cinder, heat, swift).

Default to project admin when not asked specifically for a system one.

Also, improve handling of scopes with credential "types".

Related-Issue: PRODX-55335
Change-Id: I1fcc60109cfcf2f034c33e6faac646f193685226
1 file changed