Persistence Unit mapping-file ignored by CmpJpaConversion

The CmpJpaConversion code enables TomEE to run CMP entity beans by converting them to JPA "on the fly". It does this by using a persistence unit it creates on the app module named "cmp", and generating an orm.xml file for the persistence component to use.

However, if you create a persistence.xml file and a custom orm.xml file (using the `openejb.descriptors.output=true` system property, for example) and reference the custom orm.xml in persistence.xml, it is ignored.

One possible workaround is to remove the mapping file reference from persistence.xml and package it in the artifact as META-INF/openejb-cmp-orm.xml (or WEB-INF/openejb-cmp-orm.xml for .war files): https://github.com/apache/tomee/blob/0165264d37f9cb6ce6d2585f2f6b0fbd926ad5c4/container/openejb-core/src/main/java/org/apache/openejb/config/ReadDescriptors.java#L553 and https://github.com/apache/tomee/blob/0165264d37f9cb6ce6d2585f2f6b0fbd926ad5c4/arquillian/arquillian-tomee-tests/arquillian-tomee-webprofile-tests/src/test/java/org/apache/openejb/arquillian/tests/cmp/CmpMappingTest.java#L45

However, it should be possible to also check for the presence of an existing mapping file on the cmp persistence unit, and use that as opposed to generating a new one.