dashboard: append mgr modules to ceph_mgr_modules
dashboard: append mgr modules to ceph_mgr_modules

when `dashboard_enabled` is `True`, let's append `dashboard` and
`prometheus` modules to `ceph_mgr_modules` so they are automatically
loaded.

Closes: #4026

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible