Fix QA import of existing instances

Commit f346a7d9 broke the TestInstanceImport test in QA, since it
dropped the "always generate" parameter for backup import. We add an
override mac parameter so that, no matter what is defined in the
config file, imported instances (can) have a different MAC.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>