kiji-rest: REST-97. Update Python client to recent KijiREST changes.

 - Remove ability to set explicit list of instances.
 - KijiREST admin start waits for ping to reply.

issue: https://jira.kiji.org/browse/REST-97
review: https://review.kiji.org/r/1612
commit: https://github.com/kijiproject/kiji-rest/commit/e2f21b5ade763c18ebdceefee05ceb05a8f7ae92