Merge lp://qastaging/~rvb/maas/no-dns-config-pregeneration into lp://qastaging/~maas-committers/maas/trunk

Drop the pre-population of the zone files with generated hostnames.

Back when the DNS work got started, we decided to pre-fill the forward and reverse zone files with mappings from all the IPs in the network to hostnames generated from these IP addresses (e.g. 192-168-10-253.<domain> <--> 192.168.10.253); This was a simplification as it meant the hostnames for real nodes would simply need to be translated into CNAME records on top of the pre-filled A/PTR structure (i.e. no update of the reverse zone files). Now that we're adding support for IPv6, this simplification is making things more complex as it's forcing us to manipulate IP addresses in a way that is IPv4-specific. We've been talking about getting rid of this pre-population for a while now so I thought it was a good occasion to do it and make the IPv6 work most simple (i.e. kill two birds with one stone).

See https://docs.google.com/a/canonical.com/document/d/128QR_3ANyWIY6_K6YG2WzAJitl_Aevt23S28HjastiE/edit?usp=drive_web for a more detailed description of this change.

This was tested on my NUCs and in the CI lab.