kiji-schema: SCHEMA-903. Update AbstractCassandraKijiIntegrationTest key for external cluster.

issue: https://jira.kiji.org/browse/SCHEMA-903
review: https://review.kiji.org/r/2095
commit: https://github.com/kijiproject/kiji-schema/commit/6acc1faea602fa6e552a0e8fcb7bbfbcde962ef7