Make use of --cache option delivered with latest debootstrap.

[lueschem]

Recently debootstrap introduced the --cache option. This is also useful for edi to speed up the bootstrapping process.

Things to consider:

• edi needs to detect if debootstrap is recent enough to make use of the --cache option.
• If available, then edi can by default make use of it.
• The default caching path could go to /var/lib/edi/cache/debootstrap.
• It shall be possible to overrule the cache path (make sure it gets turned into an absolute path).
• It shall be possible to turn the cache off by providing an empty path (or cache: None).
• The caching settings shall be part of the bootstrap section:

  bootstrap:
  cache: /path/to/my/cache