Add new API to view current scheduled jobs

[dbbaughe]

From: opendistro-for-elasticsearch/job-scheduler#73

Currently it's quite hard to debug something wrong in job scheduler - such as a job not running when everything pointing to it should be running.

I'd like to see an API that returns all information about the current in-memory jobs along with the time they'll be executed next/how much time is left.

Perhaps even an API to forcefully reschedule a job? I had to allocate shards to a new node to force this behavior. Ideally this shouldn't be needed but having it available as an admin tool would be nice.