Add dashboard config api section to override host/port

By default, sensu dashboard takes the api host and port from the api configuration file.  

If the dashboard and api are running on the same box, and you use a web proxy, then the api config will have 'host: localhost'.  However, this will obviously break for the dashboard when run remotely.

This patch allows you to specify an explicit api host and port for the dashboard to connect to, e.g.:

```JSON
{
  "dashboard": {
    "bind": "127.0.0.1",
    "port": 8080,
    "api": {
      "host": "api.web-proxy.example.com", 
      "port": 80
    }
  }
}
```

No change in previous behaviour should be expected if this api subsection isn't defined.