@Override
  public String getEntityConfigXMLFile() {
    return entityConfigXMLFile;
  }
Checks to see if this config object is "read only". If it is, then an
{@link IllegalStateException} is thrown.

@since GemFire 4.0
geode-core/src/main/java/org/apache/geode/admin/internal/DistributedSystemConfigImpl.java