[MINOR] Add missing properties that allows to configure kinit thread

[idzikovsky]

What changes were proposed in this pull request?

This properties allow to configure kinit thread and not documented anywhere except for the code: https://github.com/apache/incubator-livy/blob/86fc823/server/src/main/scala/org/apache/livy/LivyConf.scala#L106-L109 https://github.com/apache/incubator-livy/blob/86fc823/server/src/main/scala/org/apache/livy/server/LivyServer.scala#L106-L141

How was this patch tested?

Manual