default token format/provider handling

The Keystone server would print a warning when both the token
format and provider were set to the default.

Also, the Keystone server would not start if the format was
commented out and the provider was set to the uuid.Provider.

Fixes: bug 1204314

Change-Id: Id7db33a1f27c4986af153efc73b22db8c6a8942e
Merge "default token format/provider handling"