Add authenticate method to oauth code

Forgot to add one last method when I initially checked in
the first set of code; the authenticate method will return an
actual keystone token that the user may now use.

Also, I added some changes to other methods because the client
has been updated.

Change-Id: Ie2707689e0df1fb1bc92177f932baf23fe1ca920
Merge "Add authenticate method to oauth code"