private ArrayList<Component> hdfsComponents() {
    return newArrayList(new Component(HOST, HDFS, NAMENODE, 0l), new Component(HOST, HDFS, DATANODE, 0l));
  }
Ensures that when an upgrade is in progress, new requests are not created
to remove identities since it would interfere with the long running
upgrade.

@throws Exception
ambari-server/src/test/java/org/apache/ambari/server/controller/utilities/KerberosIdentityCleanerTest.java