Optionally deploy unbound DNS caching resolvers in lieu of
managing /etc/hosts across all hosts.
This is an opt-in optimization providing centralized DNS
resolution containers to allow intra-cluster name resolution
identical to what openstack_hosts provides with its /etc/hosts
management.
Managing /etc/hosts is very inefficient and cumbersome in large
environments with lots of hosts, so this change allows for a
quicker and more consistent deployment using centralized resolvers.
Change-Id: Ibbc1fc77aa42c1a530e2f52a5c3c4ed86d0fe8ea
d651ed70f4