design-2.1: Change OS Flavours with OS Variants

This is the terminology used by libvirt, it's shorter, clearer, and it
makes sense. :)

Also the final sentence about "possible futures" is changed to be a bit
more clear.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>