Improve control over 'hibernate.id.new_generator_mappings' setting

It would be great if Wildfly offered better control over the properties used for configuring persistence units.

For instance the 'hibernate.id.new_generator_mappings' setting has proven hard to control it currently requires one to change persistence.xml to turn it off.

Hibernate supports specifying the properties in various ways, for example via system properties, but wildfly forces the new mappings to be enabled unless the persistence.xml says to disable them.

There are some suggestions to how smarlow@redhat.com imagines this to be implemented in the forum thread: https://developer.jboss.org/thread/267613