Fix the hiveserver2 error(No valid credentials provided) connecting metastore when HADOOP_USER_NAME environment variable exists and kerberos is enabled

[Weiliang Hao]

hi, all
    I found a problem. When kerberos is enabled, if the environment variable 
HADOOP_ USER_ NAME is set, hiveserver2 will report an error（No valid 
credentials provided） when connecting to hive metastore.
    I want to fix it and have submitted the PR. Who can help me review？THX!
    jira: https://issues.apache.org/jira/browse/HIVE-26739?filter=-2
    PR: https://github.com/apache/hive/pull/3764


Weiliang Hao