Change the Node.group attribute

Currently, the Node.group attribute is the UUID of the group, as until
recently Ganeti didn't export the node group properties. Since it does
so now, we make the following changes (again apologies for a big
patch):

- we change the group attribute to be an index, similar to the way an
  Instance.pnode and snode attributes point to the parent node(s)
- on load, we read the group.uuid attribute and we use that to lookup
  the actual group index, from previously-loaded groups info
- this means that we now first read groups, then read nodes using the
  group info, and then read instances using the node info

This patch leaves a few functions showing the group index (ugly since
it's htools internal), will be converted later.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>