Add empty policy rule to get_rule_type action

Without this empty policy rule, get_rule_type will use default, which
will demand admin role or tenant_id in object. but rule_type has no
tenant_id in its body.

Change-Id: I92b1222fbcdc2efd13ca6f586cfefefc55b59189
Closes-bug: #1487324

etc/policy.json

@@ -186,6 +186,7 @@
     "create_policy_bandwidth_limit_rule": "rule:admin_only",
     "delete_policy_bandwidth_limit_rule": "rule:admin_only",
     "update_policy_bandwidth_limit_rule": "rule:admin_only",
+    "get_rule_type": "rule:regular_user",
 
     "restrict_wildcard": "(not field:rbac_policy:target_tenant=*) or rule:admin_only",
     "create_rbac_policy": "",

neutron/tests/etc/policy.json

@@ -186,6 +186,7 @@
     "create_policy_bandwidth_limit_rule": "rule:admin_only",
     "delete_policy_bandwidth_limit_rule": "rule:admin_only",
     "update_policy_bandwidth_limit_rule": "rule:admin_only",
+    "get_rule_type": "rule:regular_user",
 
     "restrict_wildcard": "(not field:rbac_policy:target_tenant=*) or rule:admin_only",
     "create_rbac_policy": "",