I think BMC_Document is the closest you´ll get... Not entirely correct, but the I can´t think of any other class better suited.
Thinking about it, what kind of attributes do you want to track for a Certificate?
In other words, do you really need a Class (for a CI) ? Would attributes on the Web Server Class do the job?
There are 3 main attributes that we would like to track.
Hostname of Certificate (or URL of Certificate)
I think the BMC_Document class would be the closest one to fit, right? The BMC_SoftwareServer class does not offer StartDate and EndDate, but BMC_Document does.