Switch package install to Python 3 for OpenStack Rocky or later.
When upgrading, remove any python-* packages that where explicitly
installated and then autoremove --purge any dependencies that are
no longer required.
This change also switches to using the cinder-manage binary in
preference to using the internal API of Cinder to query and manage
services in cinder, avoiding the need to continue to have python-cinder
installed for charm usage.
Change-Id: Ie8f7d2d7e1ef7b3065d6d9ed244e5fd05e2f613b
e31d5eb0a4