update network-y stuff for supporting ubuntu/bionic as master

On bionic, we don't have eth0 hard coded. example below, so we use `ip
route` to figure out the default ethernet interface
```
dims@kubernetes-master:~$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: ens4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1460 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 42:01:0a:80:00:23 brd ff:ff:ff:ff:ff:ff
3: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default
    link/ether 02:42:b2:4e:dd:86 brd ff:ff:ff:ff:ff:ff
```

Also, bionic uses systemd-resolver by default and adds entries in
/etc/resolv.conf that CoreDNS does not link. So follow the
recommendation in the documentation to specify resolv.conf explicitly

Author: dims

cluster/gce/util.sh

@@ -765,13 +765,23 @@ function construct-linux-kubelet-flags {
       flags+=" --kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig"
       flags+=" --register-schedulable=false"
     fi
+    if [[ "${MASTER_OS_DISTRIBUTION}" == "ubuntu" ]]; then
+      # Configure the file path for host dns configuration
+      # as ubuntu uses systemd-resolved
+      flags+=" --resolv-conf=/run/systemd/resolve/resolv.conf"
+    fi
   else # For nodes
     flags+=" ${NODE_KUBELET_TEST_ARGS:-}"
     flags+=" --bootstrap-kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig"
     flags+=" --kubeconfig=/var/lib/kubelet/kubeconfig"
     if [[ "${node_type}" == "heapster" ]]; then
         flags+=" ${HEAPSTER_KUBELET_TEST_ARGS:-}"
     fi
+    if [[ "${NODE_OS_DISTRIBUTION}" == "ubuntu" ]]; then
+      # Configure the file path for host dns configuration
+      # as ubuntu uses systemd-resolved
+      flags+=" --resolv-conf=/run/systemd/resolve/resolv.conf"
+    fi
   fi
   # Network plugin
   if [[ -n "${NETWORK_PROVIDER:-}" || -n "${NETWORK_POLICY_PROVIDER:-}" ]]; then
@@ -2925,7 +2935,7 @@ function attach-internal-master-ip() {
   echo "Setting ${name}'s aliases to '${aliases}' (added ${ip})"
   # Attach ${ip} to ${name}
   gcloud compute instances network-interfaces update "${name}" --project "${PROJECT}" --zone "${zone}" --aliases="${aliases}"
-  run-gcloud-command "${name}" "${zone}" "sudo ip route add to local ${ip}/32 dev eth0"
+  run-gcloud-command "${name}" "${zone}" 'sudo ip route add to local '${ip}'/32 dev $(ip route | grep default | awk '\''{print $5}'\'')' || true
   return $?
 }
 
@@ -2943,7 +2953,7 @@ function detach-internal-master-ip() {
   echo "Setting ${name}'s aliases to '${aliases}' (removed ${ip})"
   # Detach ${MASTER_NAME}-internal-ip from ${name}
   gcloud compute instances network-interfaces update "${name}" --project "${PROJECT}" --zone "${zone}" --aliases="${aliases}"
-  run-gcloud-command "${name}" "${zone}" "sudo ip route del to local ${ip}/32 dev eth0"
+  run-gcloud-command "${name}" "${zone}" 'sudo ip route del to local '${ip}'/32 dev $(ip route | grep default | awk '\''{print $5}'\'')' || true
   return $?
 }