kiji-schema: SCHEMA-651. cleanup deleted instances from zookeeper in /kiji-schema/instances

issue: https://jira.kiji.org/browse/SCHEMA-651
review: https://review.kiji.org/r/1634
commit: https://github.com/kijiproject/kiji-schema/commit/89daf0635f6d20ac7653a37314da20dd5444aa67