The only D-Bus implementation which supports communicating with buses in
containers is systemd's sd-bus, which unfortunately doesn't have Python
bindings (yet). So this uses the `busctl` tool here to monitor the
systemd services in the containers, and parse its output. The parser is
very limited and probably error prone. We might want to replace this
when there are Python bindings for sd-bus.

There is currently no way to recover if a service stopped unexpectedly,
except by restarting onionkit.