Retry hostname->IP: [Errno -2] Name or service not known

During cluster configuration, the hostname is getting resolved to IP,
as etcd requires IP address as listening address.

Due to connectivity flakes or delayed network inititalization, sometimes
the IP fails to be resolved to a name with following error:
```
[Errno -2] Name or service not known
```
that leads to attempt to run etcd with empty flag.

The PR adds a proper retry (up to 5 minutes) in case the connectivity
problems happens.

I considered alternatives like: `getent hosts foo`, but unfortunetelly thay
can return IPv6 that etcd is not ready for (yet).

cluster/gce/gci/configure-helper.sh

@@ -1776,7 +1776,27 @@ function start-kube-proxy {
 # $5: pod name, which should be either etcd or etcd-events
 function prepare-etcd-manifest {
   local host_name=${ETCD_HOSTNAME:-$(hostname -s)}
-  local -r host_ip=$(python3 -c "import socket;print(socket.gethostbyname(\"${host_name}\"))")
+
+  local resolve_host_script_py='
+import socket
+import time
+import sys
+
+timeout_sec=300
+
+def resolve(host):
+  for attempt in range(timeout_sec):
+    try:
+      print(socket.gethostbyname(host))
+      break
+    except Exception as e:
+      sys.stderr.write("error: resolving host %s to IP failed: %s\n" % (host, e))
+      time.sleep(1)
+      continue
+
+'
+
+  local -r host_ip=$(python3 -c "${resolve_host_script_py}"$'\n'"resolve(\"${host_name}\")")
   local etcd_cluster=""
   local cluster_state="new"
   local etcd_protocol="http"