Zone generation does not support rDNS for other subnets

[ethanwu10]

The current zone file generation assumes that all hosts are in the primary subnet, and generates rDNS PTR records off of the last octet only

dns/build-zones

Lines 96 to 100 in 5ba3b81

	def reverse_dns_records_v4(self):
	"""Return reverse IPv4 DNS records."""
	if self.ipv4:
	yield '{self.last_quad} IN PTR ' \
	'{self.hostname}.OCF.Berkeley.EDU.'.format(self=self)

This is wrong now that we have more subnets in use.

In addition, our v4 range in EWDC is a /28, so we do not have a whole zone delegated to us for rDNS - we should look into supporting rDNS there as well.

[fawaf]

Woah, ocf has more subnets now? Is that documented anywhere?