No debconf option to set up DNS forwarding

When installing MAAS, it of course acts as the DNS server for systems under its control. However, it does not configure a DNS forwarder by default, so when systems under its control ask for addresses about which it does not know, it answers "not found" rather than forwarding the requests on to a DNS server that might know. This is fixed by setting "forwarders" in /etc/bind/named.conf.options. MAAS should be able to accept this setting as a debconf option as it is installed so that it can gracefully handle this situation.