Fix osgi copy configuration tasks

This pull request does a few things:

* Leverage `resourceGenerators` to create configuration for akka-osgi
* Migrate discovery of acceptable projects with a `reference.conf` file to its own task
* remove `akka-osgi/src/main/resources/*.conf` from gitignore.

After this, you should be able to run releases without having to git clean -fdx first (at least related to the OSGi conf).  Also, the code is mostly abstracted for use in downstream projects if they wish to copy-paste.

One thing I noticed is that OSGi tests are failing right now due to cluster bundle resolution failures.  This remains unfixed, as I wasn't sure what the tests are supposed to be doing.   All other OSGi tests remain passing.