Auto 827 health check (ready for review)

This adds a `/health` endpoint to otter to check on the status of cassandra and zookeeper.

These are the results after starting up the dev vm and cassandra is not ready.
```
$ curl localhost:9000/health
{"zookeeper_state": "CONNECTED", "cassandra_failure": "ConnectionRefusedError('Connection refused',)", "healthy": false, "zookeeper": true, "cassandra_time": 0.0011789798736572266, "cassandra": false}
$ curl localhost:9000/health
{"healthy": true, "zookeeper_state": "CONNECTED", "cassandra_time": 1.1382560729980469, "zookeeper": true, "cassandra": true}
```

Problem:  Can this be gated?  Should this be a separate service instead?