Add node's nodegroup field

If a node doesn't have a node group we'll upgrade the config making it
the cluster default. Also the node add and removal operations are
changed to set/clear the node group correctly. Finally we populate the
"members" list of nodegroups on config load with  the value from the
nodes.

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