Avoid duplicate entries for the metadata server in /etc/hosts

In cases where the metadata server /etc/hosts entry was not added by the
guest environment scripts there will be no "# Added by Google" marker. As
such the script will add a second reference to the metadata if it already
exists. We should only have 1 entry. Respect the already existing entry and
do not add a second entry.

src/usr/bin/google_set_hostname

@@ -52,8 +52,11 @@ if [ -n "$new_host_name" ] && [ -n "$new_ip_address" ]; then
     echo "${new_ip_address} ${new_host_name} ${new_host_name%%.*}  # Added by Google" >> /etc/hosts
   fi
 
-  # Add an entry for reaching the metadata server in /etc/hosts.
-  echo "169.254.169.254 metadata.google.internal  # Added by Google" >> /etc/hosts
+    # Add an entry for reaching the metadata server in /etc/hosts.
+  has_metadata_entry=$(grep metadata.google.internal /etc/hosts)
+  if [ -z "$has_metadata_entry" ]; then
+      echo "169.254.169.254 metadata.google.internal  # Added by Google" >> /etc/hosts
+  fi
 fi
 
 # /sbin/dhclient-scripts in both ubuntu and centos have some problems for us: