Add support for providing nvdDatafeedUrl to OWASP

This allows you to point to a mirror that is faster and doesn’t require an API key.

This is kind of painful to make work in ant because you can't specify the property at all if you want to use the API and I couldn't find a way to get ant to conditionally supply the property without having a dedicated invocation of the dependency-check task with/without the parameter nvdDataFeedUrl specified.