gnt-network info currently displays the connected group output unformatted:

 # gnt-network info
 ...
   connected to node groups:
    ['med', 'bridged', 'staging']

This patch adds formatting to the node group output.

Signed-off-by: Apollon Oikonomopoulos <apoikos@gmail.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>