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, the database
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.
it now does

```json
    "pulp_db_connection": {
        "connected": true
    },
    "pulp_messaging_connection": {
        "connected": true
    },
    "pulp_version": {
        "platform_version": "2.6.0"
    }

```