Fix keystoneclient auth_token middleware changes

Things changed, and the configuration wasn't read anymore. This patch fixes
that. It also remove duplicated code in acl.py, so the problem is fixed only
in one place. Finally, it uses prepare_service() to find the right
configuration file for ceilometer.

This fixes bug #1098204

Change-Id: I0d6c30ad443a4d0db201e60d12b275625a4bee6e
Signed-off-by: Julien Danjou <julien@danjou.info>