Fix QA with disabled file storage

Currently QA tries to use “gnt-node list-storage” with all storage
types, but when file storage is disabled this will fail.

The patch changes it so that if file storage is disabled, we test that
indeed fails, and otherwise we include it in the regular tests.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>