Using GitLab's official backup tool takes too much time and, more
importantly, space; /srv/gitlab is a bit over 430G but backing it
up nearly exhausts its 1TB volume.

As we're creating btrfs snapshots and backing those up with borg, it
seems unnecessary to also create tarballs of the same data. GitLab's
documentation mentions snapshots as a viable backup strategy, and to
the restored system it should seem like recovering from a power loss.

[1] https://docs.gitlab.com/ee/raketasks/backup_restore#alternative-backup-strategies

This is meant as a internal authenticated and encrypted network which we
can use for internal services, we don't want to expose to the internet
or when encryption is desired but not easily implementable.

Zabbix has been replaced by Prometheus for monitoring our services.

Added missing hosts file for gitlab.archlinux.org to make sure the playbook
runs correctly.

Signed-off-by: Florian Pritz <bluewind@xinu.at>