Allow Proxy Users in Hive Metastore Client

We should allow proxy users in the Hive Metastore client.  Proxy user impersonation should work for Hive just as it is working for HDFS

Currently the client uses

UserGroupInformation.getLoginUser().getRealUser().doAs

which results in a NullPointerException when a proxy user is used, as `getRealUser()` returns `null`. 

Allowing the current user to only requires dropping `getRealUser()` for the `doAs()`. foo 

UserGroupInformation.getLoginUser().doAs