This was previously monitored as part of the borg text collector, but
now that it only runs after each backup (instead of hourly) the stats
from monitoring.archlinux.org do not remain accurate for long. Switch
back to hourly checks of the storage box's disk usage by adding a new
text collector just for this purpose.

Add a new role called prometheus_exporters which should be run on every
machine we have and starts different collectors depending on what group
the machine is in. Currently supported our the gitlab runner exporter,
rebuilder textcollector, mysqld-exporter, borg textcollector and an
node/arch exporter. The arch exporter monitors the security status and
pacman out of date packages gauge.