code: public boolean hasDeploymentSubsystemModel(final String subsystemName) {
          final Resource root = deploymentUnit.getAttachment(DEPLOYMENT_RESOURCE);
          final PathElement subsystem = PathElement.pathElement(SUBSYSTEM, subsystemName);
          return root.hasChild(subsystem);
      }
docs: Checks to see if a subsystem resource has already been registered for the deployment.
  
  @param subsystemName the name of the subsystem
  
  @return {@code true} if the subsystem exists on the deployment otherwise {@code false}