private String getDefaultRealm(Map<String, Map<String, String>> configurations) {
    return getValueFromConfiguration(configurations, KERBEROS_ENV, DEFAULT_REALM);
  }
Safely retrieves the <code>realm</code> property of the <code>kerberos-env</code> configuration.

@param configurations the existing configurations for the cluster
@return the requested value or null if the configuration does not exist
@see #getValueFromConfiguration(Map, String, String)
ambari-server/src/main/java/org/apache/ambari/server/serveraction/upgrades/PreconfigureKerberosAction.java