Secure LLAP configurations are still deemed unsecure in Tez AM processes

Due to the change in HIVE-22354 and HIVE-22195 Zookeeper discovery of LLAP workers is not working when invoked from within a Tez AM process: a Tez AM process does not log on using Kerberos even in secure environments, hence

 UserGroupInformation.getLoginUser().hasKerberosCredentials() 

will return false for security-enabled clusters too.