cloud-init creates /etc/NetworkManager/conf.d/99-cloud-init.conf,
for NetworkManager to not update resolv.conf. However, subsequent
update of NetworkManager seems to remove that file as there is no
config(noreplace) directive for it. So after an update/upgrade of
overcloud nodes, reboot would cleanup the dns entries.
cloud-init do configuration only one time during first instance
boot. So this would ensure resolv.conf is not touched during