Use roslaunch arg resolution in rosparam text

Would be very useful to do things like

```
rosparam param="app_lists">$(arg app_lists)</rosparam>
```

or even use `$(find ...)` embedded in the raw yaml. This works for `<param.../>` where the value is a tag attribute.  Obvious advantage with rosparam is to be able to parse lists and dictionaries. Over the last three years I keep running into this and seem to be finding more situations where it's desirable now I'm using args alot more.

The fix here is a simple one-liner. It works, but is there intentionally a reason this wasn't implemented?