Readd CentOS 7 with conditions

Readd CentOS 7 with conditions

The CentOS 7 distribution could still be used be deploying ceph if
  - it's a containerized deployment
  - it's a non containerized deployment without the dashboard (due to
missing python3 libraries).

The ceph_stable_redhat_distro variable has been remove because we can
rely on the ansible_distribution_major_version fact instead.

The copr el8 repository configuration is only applied for CentOS 8.

The ceph-mgr-dashboard package is only installed when the
dashboard_enabled variable is set to true.

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>