FORGE-1670: JPA Setup Command should allow explicitly overwriting existi...


Note: I added a method to the `PersistenceOperations` that returns the `PersistenceUnitCommon` instance for the given name in the given project. 
Note2: I have changed `PersistenceOperationsImpl.setup()` to update the PU if some with the given name already exists. Till now it created a completely new PU regardless of the fact that one with the given name was already present (that probably broke the specification ("Each persistence unit must be identified with a name that is unique to the persistence unit’s scope.")). Was that a right move?

If the PR is ok, I can add tests to the `JPASetupWizardTest` testing the rewriting of the persistance unit.