SubordinationManager does not always select JTA/JTAX correctly

SubordinationManager return an implementation of the Subordinate transaction support interfaces depending on if the system is running in JTA or JTAX mode. It chooses the right one when running standalone, but gets it wrong when embedded in AS or running some non-standard config. Improve the mechanism it uses to make a decision on which version to supply and also add setter to allow explicit programmatic override.