@Override
  public Configuration getConf() {
    return this.conf;
  }
This is a VisibilityLabelService where labels in Mutation's visibility expression will be
persisted as Strings itself rather than ordinals in 'labels' table. Also there is no need to add
labels to the system, prior to using them in Mutations/Authorizations.
hbase-server/src/test/java/org/apache/hadoop/hbase/security/visibility/ExpAsStringVisibilityLabelServiceImpl.java