Use central agent manager's keystone token in discoveries

Currently, EndpointDiscovery creates its own keystone client.
This patch passes an agent-manager reference into the discoveries,
enabling them to reuse the manager's keystone client and possibly
other things in the future.

Change-Id: I9875f15c4be25c2ac125458caa9a09927afdf3b2
Closes-bug: #1368125
Merge "Use central agent manager's keystone token in discoveries"