lxc-debian: don't hardcode valid releases

lxc-debian: don't hardcode valid releases

This avoids the dance of updating the list of valid releases every time
Debian makes a new release.

It also fixes the following bug: even though lxc-debian will default to
creating containers of the latest stable by querying the archive, it
won't allow you to explicitly request `stable` because the current list
of valid releases don't include it.

Last, but not least, avoid hitting the mirror in the case the desired
release is one of the ones we know will always be there, i.e. stable,
testing, sid, and unstable.

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