Service client auth in listener

To avoid re-authorizing on every use of service clients (nova, glance etc) elasticsearch/plugins/openstack_clients.py attaches an authorized instance of each client to the client_cache dictionary. Over time, keystone tokens will become invalid and this will result in 401s once the listener's been running for a while. I'm filing this as a reminder to myself to a) confirm it happens (it was a problem with the kilo summit demo code) and b) fix it.

There's a decorator in openstack_clients 'clear_cache_on_unauthorized' that should be attached to the serialize_xyz functions in each plugin's __init__. I'm also ok with a different approach to the problem.