Jobs that use PYTHON_CMD fails when there are spaces in python path

Commands like minify-css for mobile apps fails when you have python installed in the path containing spaces, like Program Files on Windows.

Note: Tested only on windows, but I dont see reason it should break linux builds as well, there is nothing wrong with having path wrapped in double quotes.

Here is error message:
```
----------------------------------------------------------------------------
    Executing: minify-css
----------------------------------------------------------------------------
>>> Executing shell command "C:\Program Files\Python27\python.exe C:\Users\Michal\Workspace\mobileapp-poc\qooxdoo-sdk/tool/bin/mkdir.py ./build/resource/mobileapp/css"...
'C:\Program' is not recognized as an internal or external command,  program or batch file.
<type 'exceptions.RuntimeError'> : Shell command returned error code: 1
```