htools: fix potential bug in ialloc/change-group

Currently, the ChangeAll mode of nodeEvac computes the primary group
of the instance and then uses the resulting group index for computing
the group score. However, during the change-group operation (which
also uses ChangeAll), the group of the instance at the beginning and
end of the operation differs, so we can't do that and we must instead
pass the final group to the function.

This patch does that, by passing the group from the caller of
nodeEvac.

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