Bind source address for cluster communication

The first address specified as a bind parameter
(server.bindaddr[0]) gets used as the source IP
for cluster communication.

If no bind address is specified by the user, the
behavior is unchanged.

This patch allows multiple Redis Cluster instances
to communicate when running on the same interface
of the same host.
Merge pull request #1567 from mattsta/fix-cluster-join

Bind source address for cluster communication