bootstrap: Introduce post download SHA1 check

Sometimes we might be on a flaky internet connection and
our JeOS download might end up corrupted. Let's do a SHA1
check after the image is downloaded. If running in
interactive mode, offer the user to download it again,
otherwise, just log that the file is corrupted and
proceed.

This way when the subsequent 7zip command fails, it's
apparent why it has failed (corrupted file).

CC: Ademar Reis <areis@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>