Loadbalancer: Use balance => source for connection that have session.

In order to avoid the Connection Reset By Peer error, (ie. #561),
we use balancer => source for every connection that involves session.

Currently, spice_cluster and glance_api_cluster.
Merge pull request #587 from enovance/bug/561/spredzy

Loadbalancer: Use balance => source for connection that have session.