Disallow non-admin users update net's shared attribute

Currently non-admin user cannot create a network with
shared=True. But the user can create the network and then
change the shared attribute to True.

This patch will no longer allow non-admin user to update a
network's shared value to True.

Change-Id: Id596ee399c56b9882efab97a89dbf7d14c5cf7f4
Closes-Bug: 1268823

etc/policy.json

@@ -35,6 +35,7 @@
     "create_network:provider:segmentation_id": "rule:admin_only",
     "update_network": "rule:admin_or_owner",
     "update_network:segments": "rule:admin_only",
+    "update_network:shared": "rule:admin_only",
     "update_network:provider:network_type": "rule:admin_only",
     "update_network:provider:physical_network": "rule:admin_only",
     "update_network:provider:segmentation_id": "rule:admin_only",