When ceph-ansible is used, it tends to use `ceph` group for ceph.conf[1]
This make problems for libvirt and cinder/ceph
as their users are not in ceph group, so don't have access to configs.
This also updates ceph_stable_release to nautilus
[1] bc701860d5/roles/ceph-config/tasks/main.yml (L66-L94)
Change-Id: I120a6e66351db62bbd6e270495f455a5e34b4a2b