Allow to deploy more than one DNS server - master/slaves

Because chef-clients could generate configuration in different times and
it can take some time, only master/slave replication will provide consistency
for all DNS zones on each nodes.
allow for 7 DNS servers

This number is arbitrary, but
7 DNS servers ought to be enough for everyone
as more will incur overhead for syncing zones
and even big websites like facebook, wikipedia, google and microsoft
only use 2, 3, 4 and 5 DNS servers.
notify all slave servers of zone changes

normal behaviour is to only notify servers listed in NS records
which for many zones is only the master DNS server