Livy proxy user + kerberos cannot support long running applications

Because of the limitation of Spark, currently if Livy is running with proxy user + kerberos environment, it will fail to get new tokens after TTL, which means session will be failed after 7 days by default.

This is mainly due to the problem that proxy user and principal/keytab cannot be coexisted, and without principal/keytab, Spark lacks the ability to re-issue tokens. So inside Spark it should have a new way to issue tokens rather than binding to principal/key on hand. Also in Livy side we should figure out a way to pass new tokens to Spark.