allow ad-hoc config parameters for transaction system

The AS config model currently exposes around a dozen config properties for the transaction system, out of a possible set of over 100 in JBossTS. Most of the others don't normally need to be changed, but there are exceptions to every rule. To support these edge cases the configuration system should allow additional properties to be specified. This is currently possible by setting system properties before starting the jvm, but not through the server's own config system. The jboss-as-txn schema should be extended to support a <properties> section that can be used in preference to system properties for overriding config values that are not individually exposed.