[Feature] Add one-shot systemd service to update /etc/resolv.conf in the instance

[praveenkumar]

Originally posted by @cfergeau in #4145 (comment)

Have you explored doing something similar to the suggestion in https://access.redhat.com/solutions/6250271 ?

The file /etc/resolv.conf can be modified using the nmcli utility via a one-shot systemd service that is (executed after ovs-configuration.service)

This would avoid the need for manually starting anything systemd related. Maybe we can unconditionally overwrite resolv.conf, and upload the one shot unit file, and that's it, no need for sd.Status or sd.Start