Change keystone tenant and user default

Currently, the default auth tenant and user for
glance are set to admin. This is not only
inconsistent with the rest of the modules (where
the tenant defaults to services and the user defaults
to the name of the service: ie: glance), it
is also inconsistent with the glance::keystone::auth
class.

This patch changes the defaults to services and glance
for both the api and registry service.

Change-Id: I3bd6390183c88213ec4bc289ca6be6f2d9c8bdd2