code: public boolean isDatasource(String datasourceName) throws Exception {
          Address addr = Address.root().add(SUBSYSTEM, SUBSYSTEM_DATASOURCES);
          String haystack = DATA_SOURCE;
          return null != findNodeInList(addr, haystack, datasourceName);
      }
docs: Checks to see if there is already a datasource with the given name.
  
  @param datasourceName the name to check
  @return true if there is a datasource with the given name already in existence
  @throws Exception any error