update GCE instance image with Debian 8 Jessie (#20558)

* update instance image with Debian 8 Jessie

* debian-7 is marked as "DEPRECATED" in Google Compute Engine Images
  * as a result, by default use `debian-8` Jessie

Resolves:
Related:
Signed-off-by: Daniel Andrei Minca <danielandrei.minca@gameloft.com>

* update docs regarding latest stable default image

* after speaking with @gundalow, we decided it's better for the users to
  know that the default image will follow latest stable debian by GCE

Resolves: #20558
Related:
Signed-off-by: Daniel Andrei Minca <danielandrei.minca@gameloft.com>

lib/ansible/modules/cloud/google/gce.py

@@ -33,9 +33,10 @@ description:
 options:
   image:
     description:
-       - image string to use for the instance
+      - image string to use for the instance (default will follow latest
+        stable debian image)
     required: false
-    default: "debian-7"
+    default: "debian-8"
   instance_names:
     description:
       - a comma-separated list of instance names to create or destroy
@@ -617,7 +618,7 @@ def change_instance_state(module, gce, instance_names, number, zone_name, state)
 def main():
     module = AnsibleModule(
         argument_spec = dict(
-            image = dict(default='debian-7'),
+            image = dict(default='debian-8'),
             instance_names = dict(),
             machine_type = dict(default='n1-standard-1'),
             metadata = dict(),