Switch Galera HAProxy to active-passive HA

Change the HAProxy configuration for Galera to an active-passive model
due to compatibility issues with OpenStack SQL statements and Galera
configured with multiple writers. With this configuration HAProxy will
only write to a single Galera node at any given time, rather than
distributing writes across all nodes.

Partial-Bug: #1321734
(cherry picked from commit 140b84c3f1f9d8d1f4179ed915e79945782c830f)
Merge pull request #20 from CiscoSystems/bugs/1321734

Switch Galera HAProxy to active-passive HA