cli.GetOnlineNodes: Support node group filter, use query2

This patc changes cli.GetOnlineNodes to use query2, which does the
filtering in the master daemon, and adds a new parameter to filter by
node group.

Unittests were added for the old implementation and then adopted to
ensure no functionality was lost.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>