Create new update site zip(s) (rather than target platform zip) for JBT content needed for Central/EA site

In order to simplify the process for getting JBT features into Central (for JBDS) and EA (for both JBT/JBDS), it's been suggested (by manderse@redhat.com, who doesn't like opening JIRAs ) that instead of a target platform which needs to be updated every time we spin or respin a build, we could instead just produce an update site containing the handful of IUs (cordovasim, arquillian, etc.).

We could further extend this idea to include all the things that currently are in BOTH JBT or JBDS and in Central, such as TestNG.

These would be produced as new zips using the JBT aggregate builder:

https://github.com/jbosstools/jbosstools-build-sites/tree/master/aggregate/

Need to decide what to call this... "Extras" "JBT-not-in-JBDS" "Shared" ...?