This patch creates a new middleware context to retrieve data
from subordinate charm and update the kesytone configuration file.
It also allows integration with keystone-middleware interface:
https://github.com/openstack-charmers/interface-keystone-middleware
This patch uses the subordinate configuration approach
to retrieve data from the subordinate charm.
Every changes required for paste.ini file will be handled by
the subordinate charm. The latter should deal with keystone upgrades.
Closes-Bug: #1808597
Change-Id: I4897011fbc791abc97e34e75826579820e80a4f1
c272d2c707