The dns_domain attribute of a network is intended for use
by neutron when creating DNS records in an external DNS
system such as Designate.
By using the networks dns_domain, the configured search
path on booted instances mismatches with the generated
dns assignments for instance ports in the hosts file
for dnsmasq which creates a mismatched forward/reverse
lookup behaviour.
This reverts commit 7fdd6adc7a.
and commit 137a6d6105.
Closes-Bug: 1826419
Depends-On: I145144c042b100f7e12a02a8ac7e0fbbe41e984d
Change-Id: I5ff03b5ad8af432a9f7919ef953d7d8c434b93bd