Move dns.reversename import to head of models.py
Move dns.reversename import to head of models.py

It is necessary because we use this function it the
pretty_ipv6_ptr don't turned on.
ngoduykhanh/PowerDNS-Admin,ngoduykhanh/PowerDNS-Admin,ngoduykhanh/PowerDNS-Admin,ngoduykhanh/PowerDNS-Admin