Remove .debs from the instance filesystem

Remove .debs from the instance filesystem

This patch removes the .debs which were used in the debootstrap process
via a find and xargs invocation (as opposed to chroot $TMPDIR apt-get
clean).

The main reason for this is that:
  - we don't want the created cache to contain the debs (it's
    unreasonably larger than needed)
  - but we want an instance created with the cache to be the same as
    without cache

The patch also adds ‘-f’ to the rm and mv invocations which didn't have
it.

Reviewed-by: imsnah