Add cli option to rebuild chroot for specific build environment

[convolu]

Sometimes it is useful to be able to regenerate it.

The chroot for the supported build environments is being generated at the creation of the repo and stored as a gzipped tarball in the repository directory.

Currently the only way to regenerate it is to delete the tarball. It may be useful to have a cli interface to doing this.

[pazdera]

That's a good idea!