Handle the result of QueryGroups() correctly

If no group is given for the “gnt-network connect“/“… disconnect”
commands, the client uses the result of “QueryGroups()” which is a list
of lists. Use “itertools.chain()” to handle the return value correctly.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
(cherry picked from commit af3aea48)