Tests: Add 'preferred masters' failover test

See: e90d0c585dde96a780dbd1f4109d03dba651b9c2
See: 98d74321f5d7c2f54d5faaba85f379b2a572c35c
See: d8a6072cbaf5ef66b5229035c6a60c97b3509347
See: ARAKOON-360
See: http://jira.incubaid.com/browse/ARAKOON-360