Unify Kerberos credentials checking

Inspired by the discussion in http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Yarn-Kerberos-issue-td31894.html#a31933

Currently the security HadoopModule handles delegation token login utilizes 2 different code path. 

Flink needs to to ensure delegation token is also a valid format of credential when launching YARN context. See [1] https://github.com/apache/flink/blob/master/flink-yarn/src/main/java/org/apache/flink/yarn/YarnClusterDescriptor.java#L484 and [2] https://github.com/apache/flink/blob/master/flink-runtime/src/main/java/org/apache/flink/runtime/security/modules/HadoopModule.java#L146