Allow persistence descriptors other than META-INF/persistence.xml for OpenJPA tools

OSGi persistence bundles allow JPA persistence descriptors to be in locations other than META-INF, and has no restriction on the name of the xml file. It would therefore be much easier to use the OpenJPA tools (for example the enhancer, mapping tool, etc) if they allowed a configuration parameter specifying the name and location of the persistence descriptor (for example OSGI-INF/jpa.xml).

The current default would not need to be changed, but a configuration parameter would allow for the tools to be used much more easily in OSGi application development.