Refactor shared code

We've some work which we need to do for the multi allocation request as
well, so lets refactor it to separate functions.

_ComputeNics is kinda ugly, as it reserves the MAC. I added a TODO to
think more about how we factor this behaviour out.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>