Add variables for active-passive Galera HA proxy

Add variables needed to configure Galera HAProxy in an active-passive
model due to compatibility issues between Galera configured with
multiple active writers and certain SQL statements used in OpenStack
code.

Change-Id: Iab9693c3f8a223c0a8fe730de40602892f4460b6
Partial-Bug: #1321734
Merge pull request #84 from CiscoSystems/compressed_ha_1321734

Add variables for active-passive Galera HA proxy