gnt-network: Fix CLI issues with node groups

The original implementation reserved the name “all” to specify all node
groups. That would prevent the use of a node group actually named “all”.

This patch changes the order and definition of the command line
arguments for “gnt-network connect” and “… disconnect” to have the
group(s) as the last argument(s). If no groups are specified all groups
are used.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>