feat: allow overriding of persistence properties

Prior to this change, the only way to change the persistence settings
was to edit the `persistence.xml`. In case of a pre-build .jar, the
`persistence.xml` has to be contained in the .jar with the persistence
module [1], and can not be overridden. However, the current repo
hard-codes the DB connection details in the persistence.xml, which
makes it impossible to override the DB connection details for a fully
built .jar without changing that .jar file.

This change enable the overriding of the properties used for the JPA
`EntityManagerFactory` with system properties, so that DB connection
details can be altered without changing the .jar file.

[1] https://openjpa.apache.org/builds/1.0.2/apache-openjpa-1.0.2/docs/manual/ref_guide_conf_specify.html

app/jpa/manager/impl/HibernateManager.java

@@ -38,7 +38,8 @@ public class HibernateManager implements JPAManager {
     private final EntityManagerFactory entityManagerFactory;
 
     public HibernateManager() {
-        entityManagerFactory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
+        entityManagerFactory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME,
+            System.getProperties());
     }
 
     @Override