Switch default debootstrap tool to mmdebstrap

[zeha]

mmdebstrap is the modern, fast tool. grml-small comes with mmdebstrap but not debootstrap, so it's really necessary there.

Closes: #290

[zeha]

I think we should also adjust README.md:

-This tool is a wrapper suite around debootstrap and cdebootstrap
+This tool is a wrapper suite around debootstrap and mmdebstrap

and also adapt grml-debootstrap.8.txt accordingly?

Indeed, done.

I have left the parts mentioning just debootstrap alone for now, we can continue the discussion in #209.