Updates to /status API

This patch returns additional data from the /status API call. Specifically, it
returns a worker list, the message broker connection status, and the Pulp
version (RHBZ #704184).

There is a small behavior change to include the resource manager and scheduler
as part of the worker heartbeat list. Previously, these were not included in
the list.

The status API json document does not include the Mongo connection status. Pulp
will not complete startup if Mongo is not available.