Add clean-chroots script, service and timer to devtools
Cleaning up chroots is done automatically on our main build server, but this might benefit normal users of devtools as well. As @foxboron and @freswa had to use it as well.
The benefit is that the script is better curated in devtools, and well maintained. It does need to be ported to bash from zsh and timer adjusted to be less aggressive (cleaning hourly is a bit over the top)