In hiera backend, add ability to use "deep" _nodequery

Hey!

It would be convenient if the hiera backend was able to substitute a node query when returning a structure from hiera. For example, if we have this:

``` yaml
something:
  sources::_nodequery: "Class[...]"
```

It would be convient if the hiera backend was able to return:

``` ruby
{ "something" => { "sources" => [ "host1", "host2" ] } }
```
