Enhance cli.GetOnlineNodes query/filtering

This patch allows GetOnlineNodes to return the secondary IPs instead of
the node names, and to provide filtering of the master node (required to
be done in this function in case we return the secondary IPs).

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