The `python` in `simple` should be set using `update-alternatives`.

[veritas9872]

Right now, it replies on PATH configurations from the image and the .zshrc file.
However, setting it using update-alternatives would make it stick, even when sudo bash is used.
Also, it would be nice to have somewhere that uses the update-alternatives command so that I can remember.

[veritas9872]

As mentioned in #224, I think that I should make the simple service much more general purpose.