kiji-rest: REST-89. Refresh instances automatically.

Also removes instance visibility as a configuration parameter.
All instances on target cluster are now visible.
Manual refresh can be triggered with a POST to the admin task
/tasks/refresh_instances.

issue: https://jira.kiji.org/browse/REST-89
review: https://review.kiji.org/r/1593
commit: https://github.com/kijiproject/kiji-rest/commit/3ca2ebb55e3b6ec2106e30c909930bb97c0d60c2