Add a ManagementResourceRegistration.registerSubModelIfAbsent method

Subsystems, particularly datasources and resource-adapters, have code like this that's called from service start methods:

https://github.com/wildfly/wildfly/blob/master/connector/src/main/java/org/jboss/as/connector/services/datasources/statistics/DataSourceStatisticsService.java#L84-L86

This is basically trying for a quasi putIfAbsent semantic. But it's not really reliable, particularly if multiple services are doing the same thing during boot.

So, add a proper putIfAbsent-type submodel registration method to ManagementResourceRegistration.