OPENDJ-2247: Improve CertAndKeyGen class location logic

This is a small fix/cleanup to Platform.java CertAndKeyGen handling. I've been using this to allow me to test various issues with IBM SDK 8.

The various if/then/else constructs were starting to look ugly so I changed this to just check a list of known/possible locations. There is also a java property introduced that could allow it to be easily overridden in the future.

This probably isn't the best longterm solution (I imagine the location/availability of this class could easily change again) - but it means at least DJ will be compatible with IBM SDK 8 in the meantime.