When a host is down, playbook execution will fail

[zmc]

Need to document how to work around this. We don't want the user to just remove the host from the inventory (unless it's a permanent change), since it will be missing from prometheus.yml.

Edit: an example of what I just did to deploy fresh dashboards while an OSD host was down:

ansible-playbook -v -i rhs_demo playbook.yml -l ceph-grafana -t dashboards