text: Title: Expand environment variables in argumentfile
  Body: My request is to enable the expansion of environment variables inside of the argumentfile. 
  
  **Background**: when RF reads additional path-like arguments from an argumentfile, arguments like `--pythonpath`  or `--output` are hardcoded. There is currently no way to make them dynamic. 
  
  Writing this 
  
      --output $TESTBASEDIR/somefolder/output.xml
  
  inside of an argumentfile would be awesome.
  Thanks!