install network-checks's dependencies

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 189dd6000ee37350342d246a910e1da8b384fb45 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