Add transaction subsystem object-store config to standalone-openshift.xml

Our standalone-openshift.xml files should include this element that's part of the standard EAP bare metal configs:

<object-store path="tx-object-store" relative-to="jboss.server.data.dir"/>

Not having it doesn't harm anything, since if it's not configured some complex default logic results in those settings anyway. But having it in the config makes the effective configuration clearer, plus it removes an unnecessary diff between the bare metal configs and the OS ones.