Updates _cleanup_users_resources to use imap instead of map

Updates _cleanup_users_resources to use imap instead of map so
that it only instantiates openstack clients right before
cleaning up users instead of instantiating them up front.
This is needed to avoid keystone token timeouts in case
of many users.

Change-Id: I857a14edb3932af776959539151a9997dd939c0d
Closes-Bug:#1284110
Merge "Updates _cleanup_users_resources to use imap instead of map"