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}
```

Note - this health check only checks that the node can connect to a single cassandra instance.  This doesn't actually check to make sure that the keyspace is set up correctly, or that it can connect to any other cassandra node, or that the nodes can see each other.