Enable configuration of multiple preferred masters

This commit adds a new configuration item, 'preferred_masters', which
works similar to the 'master' & 'preferred_master' combinations, but
allows setting a list of preferred master nodes.

See: 0c064f3eaa7f0eaf38ae78ac3fcbc4007020226d
See: ARAKOON-360
See: http://jira.incubaid.com/browse/ARAKOON-360
Extension: Add 'preferred_masters' configuration feature

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