Add API-level slider REST client

Using the slider REST API & authenticating Jersey code, write a REST client which binds to an AM then issues API calls.

• API calls to mimic those used by slider client today: get cluster state, enum, perform actions.

• Actions will be added client-side as they go in server side; they will fail if routed via YARN proxy and proxy doesn't support it or routed direct and the ws/ path isn't open to all callers.

• this initial implementation will not rebind to AMs that fail/get restarted elsewhere. That will need extension to re-enum the app id in the RM, or re-lookup in the yarn-registry, both with some backup policy.