Bug 2024826: Allow resolv prepender without default search domain

Commit e1fbf07 introduced a regression where the local nameserver was
only prepended to the list of nameservers when the NM generated
`resolv.conf` had a default search domain.

This commit fix the regex to look for nameservers rather than search
domain, and makes the script a little more robust by using start of
line anchor in regexes.

templates/common/on-prem/files/NetworkManager-resolv-prepender.yaml

@@ -69,9 +69,9 @@ contents:
                 >&2 echo "NM resolv-prepender: Prepending 'nameserver $NAMESERVER_IP' to /etc/resolv.conf (other nameservers from /var/run/NetworkManager/resolv.conf)"
                 sed -e "/Generated by/c# Generated by KNI resolv prepender NM dispatcher script" \
                     /var/run/NetworkManager/resolv.conf > /etc/resolv.tmp
-                sed -i "s/search.*/\0\nnameserver $NAMESERVER_IP/" /etc/resolv.tmp
+                sed -i "s/^nameserver.*/nameserver $NAMESERVER_IP\n\0/" /etc/resolv.tmp
                 # Make sure cluster domain is first in the search list
-                sed -i "s/search \(.*\)/search {{.DNS.Spec.BaseDomain}} \1/" /etc/resolv.tmp
+                sed -i "s/^search \(.*\)/search {{.DNS.Spec.BaseDomain}} \1/" /etc/resolv.tmp
                 # Remove duplicate cluster domain entries
                 sed -i "s/\(search {{.DNS.Spec.BaseDomain}}.*\) {{.DNS.Spec.BaseDomain}}\( .*\|$\)/\1\2/" /etc/resolv.tmp
                 # Only leave the first 3 nameservers in /etc/resolv.conf