Group operations: CLI code for add/remove/rename a group

Also, minor update to the 2.3 design doc, which was indicating the remove
operation would be `gnt-group del` and not `gnt-group remove` (the latter
being consistent with gnt-node and gnt-instance).

Signed-off-by: Adeodato Simo <dato@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>