Refactor keystone_user_role to be lazier

Currently, the entire list of user roles are
cached ASAP. Trying to pre-determine these
is very slow b/c there is no user role list
command in keystone, so you must iterate through
all tenant/user combinations to retrieve the list.

This patch makes the retrieval of all state info
lazy.

Partial-Bug: 1224179
Change-Id: I80a85f3ddf2c41fe447df5682ad31ac4df8f8b59
(cherry picked from commit a0a359801e7248b24d15b3acef3b398d5a2fb65f)