Disallow regular user to update firewall's shared attribute

Shared firewalls should only be operable by  admins.
Currently only admin can provide shared attribute at firewall creation,
so update_firewall should be consistent with that as well.

Change-Id: I093743514637824207b375d724404d51f778d012
Closes-Bug: #1323322

etc/policy.json

@@ -70,6 +70,7 @@
     "create_firewall:shared": "rule:admin_only",
     "get_firewall:shared": "rule:admin_only",
     "update_firewall": "rule:admin_or_owner",
+    "update_firewall:shared": "rule:admin_only",
     "delete_firewall": "rule:admin_or_owner",
 
     "create_firewall_policy": "",