using "primary" to describe the domain of the box / mail server is confusing when working with multiple domains.
Usually the box domain is different from the domain you want to host your mail for.
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)