Modification to retrieve external properties for JPA EMF

Add possibility to use external properties to instantiate a JPA EntityManagerFactory

Ex (for kundera properties) : 

# JPA PersistenceUnit
jpa.default=mongoPU

mongoPU.kundera.nodes="127.0.0.1"
mongoPU.kundera.port=27017
mongoPU.kundera.keyspace=MyDataStore