Update 2.3 design doc regarding node group features and behavior.

In particular:

  - introduce a "gnt-group" command to hold group-level operations.
  - ditch the concept of "default node group", except for single-group
    clusters.
  - introduce an "alloc_policy" attribute for node groups, indicating
    how they should be treated by automated allocation tools.
  - introduce a "drain" operation on node groups.
  - define iallocator modes for new instance allocation and
    inter-group moves (choosing among all groups, or providing a
    limiting list).
  - indicate and explain that changing the group of a node will be
    initially only supported for nodes that are empty.

Signed-off-by: Adeodato Simo <dato@google.com>
Signed-off-by: Balazs Lecz <leczb@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>