Do not remove upstream DNS configuraton in cleanup

This PR changes the logic of host cleanup so that the upstream DNS
configuration is not removed. This comes from the fact that by default
dev-scripts configure NetworkManager to use dnsmasq with DHCP-provided
DNS servers and removes the existing /etc/resolv.conf configuration.

This causes problems in environments configured without DHCP with DNS
servers configured manually without use of the NetworkManager. With this
PR user can use `ADDN_DNS` variable to provide their own DNS servers
that will be passed to the dnsmasq and will not be removed when
dev-scripts are cleaned up.

Even though this does not cause the changes to be completely reverted
(user will still end up with NetworkManager + dnsmasq), it does not
leave the machine with no DNS servers at all.

host_cleanup.sh

@@ -32,8 +32,6 @@ ansible-playbook \
     -b -vvv ${VM_SETUP_PATH}/teardown-playbook.yml
 
 sudo rm -rf /etc/NetworkManager/dnsmasq.d/openshift-${CLUSTER_NAME}.conf /etc/yum.repos.d/delorean*
-sudo rm -rf /etc/NetworkManager/conf.d/dnsmasq.conf
-sudo rm -rf /etc/NetworkManager/dnsmasq.d/upstream.conf
 if systemctl is-active --quiet NetworkManager; then
   sudo systemctl reload NetworkManager
 else