Ensure WSGI processes run under the 'gnocchi' group to
support restriction in permissions to /etc/gnocchi/* to
root or members of the gnocchi group.

This change also picks up changes in charms.openstack
to support this functionality, as well as fixing
misc issues with upgrades.

Change-Id: I14d865cd96b7c250ad92c8bdee74693a6eccc769
Closes-Bug: 1780490
Closes-Bug: 1779062
Related-Bug: 1781468