DNS requires extra setup for instances launched by function tests

Can the DNS setup for the manila-ganesha function tests be coded into zaza config?

In order to run manila-ganesha function tests [1] on my own [2] I have to do the following in order for the instances that get created by the test to get DNS:

juju config neutron-api enable-ml2-dns=true reverse-dns-lookup=true -m zaza-61c82ddb63fc
juju config neutron-gateway -m zaza-61c82ddb63fc dns-servers=10.5.0.2
openstack subnet set private_subnet --no-dns-nameservers

Osci must do something like that or otherwise in order to setup DNS because there are no issues running the tests from gerrit.

Without doing that setup when running tests locally, the instances launched by the tests fail to reach apt repositories so the cloud-init package doesn't get installed.

It's a bit of a dance to do the setup when running tests locally, particularly the subnet step, because private_subnet gets created shortly before the test is run.

[1]
https://opendev.org/openstack/charm-manila-ganesha
https://github.com/openstack-charmers/zaza-openstack-tests/zaza/openstack/charm_tests/manila_ganesha/tests.py

[2]
tox -e func-target bionic-train