lxc-debian: allow creating `testing` and `unstable`

lxc-debian: allow creating `testing` and `unstable`

Being able to create `testing` containers, regardless of what's the name
of the next stable, is useful in several contexts, included but not
limited to testing purposes. i.e. one won't need to explicitly switch to
`bullseye` once `buster` is released to be able to continue tracking
`testing`. While we are at it, let's also enable `unstable`, which is
exactly the same as `sid`, but there is no reason for not being able to.

Signed-off-by: Antonio Terceiro <terceiro@debian.org>