spark.history.kerberos.principal doesn't take value _HOST

spark.history.kerberos.principal doesn't understand value _HOST. 

It says failure to login for principal : spark/_HOST@realm . 

It will be helpful to take _HOST value via config file and change it with current hostname(similar to what Hive does) . This will also help to run SHS on multiple machines without hardcoding principal hostname.  .spark.history.kerberos.principal

It require minor change in HistoryServer.scala in initSecurity  method . 

Please let me know , if this request make sense , I'll create the PR .