allow users to disable "thread-pool-per-bundle"

Currently, a thread pool is created for each bundle (with threads automatically dying after a timeout). Each bundle is associated with a thread group, so that thread spawns
by a given bundle can be easily associated back to the bundle. It can also help disagnosing bundles that do not stop threads correctly.

Application with a very large number of bundles could result in a very large number of threads created at Fuse ESB Enterprise startup. In theory this could reach the "limit" for number of threads available in that particular JVM.

It would be good to have a configuration option to revert to previous thread model if the user desired it.