XD-1191b: Rewrite jdbc module options metadata as a pojo

Proposed as an alternative to https://github.com/spring-projects/spring-xd/pull/491, so that defaults still go thru module options.

Now, with the ongoing discussion about defaults, there are still issues with the choice of names for the keys in jdbc.properties IMO: url and username for example are fairly generic and e.g. a rabbitmq username could be picked up for the jdbc module if the jdbc.properties does not exist.