Update the GCE guide with a working configuration.

Also remove a few whitespaces, improve the bash script and fix a typo.

docsite/rst/guide_gce.rst

@@ -136,15 +136,15 @@ For the following use case, let's use this small shell script as a wrapper.
   #!/bin/bash
   PLAYBOOK="$1"
 
-  if [ -z $PLAYBOOK ]; then
-    echo "You need to pass a playback as argument to this script."
+  if [[ -z $PLAYBOOK ]]; then
+    echo "You need to pass a playbook as argument to this script."
     exit 1
   fi
 
   export SSL_CERT_FILE=$(pwd)/cacert.cer
   export ANSIBLE_HOST_KEY_CHECKING=False
 
-  if [ ! -f "$SSL_CERT_FILE" ]; then
+  if [[ ! -f "$SSL_CERT_FILE" ]]; then
     curl -O http://curl.haxx.se/ca/cacert.pem
   fi
 
@@ -188,11 +188,14 @@ A playbook would looks like this:
          wait_for: host={{ item.public_ip }} port=22 delay=10 timeout=60
          with_items: gce.instance_data
 
-       - name: add_host hostname={{ item.public_ip }} groupname=new_instances
+       - name: Add host to groupname
+         add_host: hostname={{ item.public_ip }} groupname=new_instances
+         with_items: gce.instance_data
 
    - name: Manage new instances
      hosts: new_instances
      connection: ssh
+     sudo: True
      roles:
        - base_configuration
        - production_server