Since it runs before the real setup begins, we must make sure that packages are installed.
Also removing bind9-host's installation from system.sh. In 189dd6000e I added this so we could use `host`
to aid Docker autoconfiguration. Docker support was since removed but this hadn't gotten removed, which lead me to think it was
normally installed by Ubuntu. It's now installed in `network-checks.sh`.
fixes#180
* check that the PUBLIC_IP is not listed in zen.spamhaus.org
* check that the PRIMARY_HOSTNAME is not listed in dbl.spamhaus.org
* check that a connection to Google's MTA is working (i.e. we're not on a residential network that blocks outbound port 25)