with many users and groups in a domain fetching all mappings (for both
users and groups) may become inefficient.
In an environment with approx 125k users and 150 groups in the mapping
table and SAML2+LDAP auth/backend, this patch reduced the time
for first (uncached) 'openstack token issue' command from 12 to 3 seconds.
Similar improvements were seen with time to login to Horizon as well.
(cherry picked from commit