In order to be able to use the group policy during allocation, we have
a few choices:

- pass the group policy as a separate argument to all involved
  functions, or
- add the policy as an attribute to nodes, or
- add the group (not only the group index) as an attribute to nodes

The first and last one would change more than wanted for this patch,
so I went with the second one; it will allow easily checking the
target policy at runtime (based on the instance's primary node), with
the only downside that without a node, we can't check the policy.

Note that the first option also would complicate the allocation
routines, which examine (big) lists of node/node pairs, so adding the
group to this would complicate things even more.

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