Globally support system scope credentials

Globally support system scope credentials

After spending huge effort to understand the exact requirements to
enforce SRBAC, we learned it's very difficult to find the required
scope in each credential. This requires understanding implementation of
client-side as well as server-side, and requirement might be different
according to the deployment architecture or features used.

Instead of implementing support based on the actual implementation,
this introduces support for system scope credentials to all places
where keystone user credential is defined, and make all credential
configurations consistent.

Change-Id: I3a659a6b43d9c47e88334c24fb866a73a8f24a24