Adding a test only mode, where nothing is checked in into SCM

Running release:prepare is scary, since the release plugin will deply modify your POMs and check them in to the repositoy, making release tags without any way to preview how POMs will be tranformed and what SCM operations will be performed.

We need a flag in order to be able to run this goal in "test mode" and let users check what will happen without any impact on the repository.