The ``creds`` dictionary passed into oslo.policy's enforce() method
assumes a lot of the same values already specified by oslo.context
RequestContext objects.
This commit teaches enforce() to handle being passed an instance of
a RequestContext object, and populate credential values accordingly.
Change-Id: Ia74bf6c40b1e05a1c958f4325e00f68be28d91b9
Closes-Bug: 1779172