download: Improve cache handling

download: Improve cache handling

This adds a new --force-cache parameter which will force use of the
cache even for expired images.

An expired image is now only flushed from the cache once a new one is
successfuly downloaded (to avoid destroying the local cache when the
host doesn't have internet connectivity).

The ID of the build in cache is also tracked so that we don't
re-download something we already have (should only happen if we don't
have a new build published by the time the previous one expires).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>