[r=julian-edwards][bug=][author=rvb] Refactor the reverse zone file writer class so that it accepts the same kind of mapping the forward zone file writer does. This is in preparation for when the mapping will be hostname→[ip1, ip2, etc.] instead of hostname→ip: the better encapsulation of how the reverse zone file deals with the mapping means the change will be easier to do. All the logic is now in the DNS-config writing utilities (src/provisioningserver/dns/config.py) and not in intermediate code such as the code in src/maasserver/dns.py.