add "required datasource" capability

I'd like to see something like this to. Which datapoints alert on no data is pretty hit and miss and I beleive only WMI alerts when the creds are wrong. Can be hard to figure out if you are polling everything needed. We've done a workaround before by editing the applies to field of a datasource to point to a group where we know the datasource should be applied and then alerting if no data. Although this may not work if AD fails to find any instances as is what looks like happend here.