Allow per-hypervisor optional files

Rather than just allowing files for all nodes to be optional, we allow
optional files to be per-category. The way this works is that they must
be included in both lists (the new code checks for this).

The code also removes a duplicate assert (present both in verify and
compute ancillary files) and checks the new functionality in unittests.

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