Tests: Add first test for 'preferred_masters'

This commit adds a first test for 'preferred_masters', by copying the
existing 'system_tests_preferred' test, and changing it slightly to use
'preferredMasters' instead of 'forceMaster' with 'preferred=True'.

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