add read_file, write_file, mkdir, adjust_permission functions in vsc.utils.missing

This is more readable if you have:  
    if os.path.exists(path):
        _log.debug("Not creating existing path %s" % path)
        return

Then the rest of the code can be reduced by an indent and you don't need to scroll around to see where the else 30 lines down matches up to. This also prevents nested if statements in the future if you need to check more behaviour. [More here](https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Coding_Style#Error_handling).