roslaunch: Moving logical expression parsing to loader

- added simple conditional parser
- disabled tests in roslaunch conditionals which we're no longer trying
  to implement
- fixing indentation of conditional test python script

This currently parses the roslaunch xml file, but the tests in the python script fail. This PR no longer supports any mathematical operations, instead it only supports the primitive operators based on the simple conditional expression parser based on https://gist.github.com/cynici/5865326

```
and or not
* ** / // %
+ -
is < <= > >= == !=
```