container: explicitly pull monitoring images

container: explicitly pull monitoring images

We don't pull the monitoring container images (alertmanager, prometheus,
node-exporter and grafana) in a dedicated task like we're doing for the
ceph container image.
This means that the container image pull is done during the start of the
systemd service.
By doing this, pulling the image behind a proxy isn't working with podman.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1995574
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>