Use default reverse zone consistently
When a new reverse zone is to be generated based on an IP address without
a network prefix length, we need to use some default value. While netaddr
library default ones (32b for IPv4 and 128b for IPv6) are not very sensible
we should use the defaults already applied in installers. That is 24b for
IPv6 and 64 for IPv6.
Test case has been added to cover the new default.
https://fedorahosted.org/freeipa/ticket/2461