Allow users to configure the persistence unit that the jBPM Designer/Consoleuses.

At the moment, the CommandDelegate code just has the following hard-coded:

emf = Persistence.createEntityManagerFactory("org.jbpm.persistence.jpa");

I have no real ideas yet about how we could make this configurable, but it would be nice to at least allow users to configure that and then rely on "org.jbpm.persistence.jpa" if the configuration failed.

(Rather than do something with jbpm.console.properties, I've been wondering if it's not possible to hook into the persistence of any ksession that we create.. or something like that).