kiji-rest: REST-101. Allow KijiREST plugins to use arbitrary k/v in the config.

Also change KijiRESTConfiguration's getClusterURI method to not be final.

issue: https://jira.kiji.org/browse/REST-101
review: https://review.kiji.org/r/1625
commit: https://github.com/kijiproject/kiji-rest/commit/1d3105ba5326dd817e97282dc6839bb488b5dbdf