Use credentials_file for GCE module auth config

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

@@ -91,7 +91,14 @@ options:
   pem_file:
     version_added: "1.6"
     description:
-      - path to the pem file associated with the service account email
+      - (deprecated) path to the pem file associated with the service account email
+    required: false
+    default: null
+    aliases: []
+  credentials_file:
+    version_added: "2.1.0"
+    description:
+      - path to the JSON file associated with the service account email
     required: false
     default: null
     aliases: []
@@ -105,7 +112,7 @@ options:
 
 requirements:
     - "python >= 2.6"
-    - "apache-libcloud >= 0.13.3"
+    - "apache-libcloud >= 0.17.0"
 author: "Eric Johnson (@erjohnso) <erjohnso@google.com>"
 '''
 
@@ -178,12 +185,13 @@ def main():
             state = dict(default='present'),
             service_account_email = dict(),
             pem_file = dict(),
+            credentials_file = dict(),
             project_id = dict(),
         )
     )
 
     if not HAS_LIBCLOUD:
-        module.exit_json(msg='libcloud with GCE support (0.13.3+) required for this module')
+        module.exit_json(msg='libcloud with GCE support (0.17.0+) required for this module')
 
     gce = gce_connect(module)