Cloud-init should support disabling ZEROCONF

[ani-sinha]

Zeroconf is described here: https://www.brennan.id.au/04-Network_Configuration.html#zeroconf
CentOS based distros support NOZEROCONF setting to disable it and kickstart sets NOZEROCONF=yes in /etc/sysconfig/network in some deployments without which the instance does not come up. However, cloud-init sysconfig renderer removes this setting as a part of https://github.com/canonical/cloud-init/blob/main/cloudinit/net/sysconfig.py#L1123 .

This issue was raised here https://bugzilla.redhat.com/show_bug.cgi?id=1653131 and we had a downstream hack to address this up until now. See also https://access.redhat.com/solutions/3958321.

I wonder if there is a better way to address this.
(a) a cloud-init native config setting to disable zeroconf would be ideal. So we cloud do

if network_state.disable_zeroconf:
   netcfg.append("NOZEROCONF=yes")

(b) If not, lets not wipe contents of /etc/sysconfig/network if it is present since it may contain necessary customizations. What we can do is read the contents and append

                netcfg.append("NETWORKING_IPV6=yes")
                netcfg.append("IPV6_AUTOCONF=no")

to it leaving existing contents as is. I will propose a patch for the second approach.