When a new group is added, if the instance communication network is
enabled, then this group must also be connected to this network.

* 'LUGroupAdd._AddInstanceCommunicationNetwork' connects the newly
  added group to the instance communication network by resorting to
  the 'OpNetworkConnect' opcode.

* As a result of the previous change, the type of 'OpGroupAdd' changed
  and may optionally return a list of jobs.  Therefore, it is
  necessary to update the type of the opcode (in 'Ganeti.OpCodes') and
  the CLI for 'gnt-group add' must also be changed.

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>