This patch basically does a simple thing, on the basis
of a variable neutron_dns_integration it enables/disables
DNS integration.

There is also precheck added which checks whether dns_domain
in neutron.conf has a non-default value if DNS integration is
enabled as this is requirement.

[1] https://docs.openstack.org/neutron/latest/admin/config-dns-int.html
[2] https://docs.openstack.org/neutron/latest/admin/config-dns-int-ext-serv.html#config-dns-int-ext-serv

Closes-Bug: #2049503

Change-Id: I90f0f8dcec6fa0112179f050d96e9d9db5956cf8