Added a policy for retrieving the agent hosting a load balancer

LBaaS V2 needs a separate policy from V1 that will allow only an admin
to retrieve the agent hosting a particular load balancer.

Change-Id: Ida9c3ce15cf14431072d5505396bca11aa92b276

etc/policy.json

@@ -115,6 +115,7 @@
     "get_loadbalancer-agent": "rule:admin_only",
     "get_loadbalancer-pools": "rule:admin_only",
     "get_agent-loadbalancers": "rule:admin_only",
+    "get_loadbalancer-hosting-agent": "rule:admin_only",
 
     "create_floatingip": "rule:regular_user",
     "create_floatingip:floating_ip_address": "rule:admin_only",