How to customize JBT to be like JBDS

In JBDS, we override the discovery.url to set it to the value for JBDS Central site.
However, this is done by hacking system properties. Instead, we could think of using the "discoveryProviderUrl" extension point in JBT.
JBT would provide an extension to define this URL.
JBDS would provide another extension to define this URL.
Changing URL happens by updating the extension plugin.

So JBT and JBDS would behave the same way when dealing with discovery, and we remove the system properties settings from JBDS. System proprty would still be available for testing only.