Create a JBDS parent pom

Having a JBDS parent pom would be very comfortable. It would inherit from JBT parent pom, but would override some properties (such as local.site) and target-platform definitions.
This is currently done in configuration in Jenkins, so it requires lots of parameters to build. Having a parent pom would allow to build JBDS with the same required parameters as JBT.

It would also help to make the JBT parent pom cleaner: a lot of complexity in JBT parent pom comes from the fact that we want to be able to override everything since we use it for multiple purposes (JBT & JBDS).
A parent pom would move this complexity to another file (JBDS parent), and won't pollute JBT.