[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

[github-actions]

This pull request has been automatically marked as stale because it has had no activity for at least 3 months. If you are still working on this change or plan to move it forward, please leave a comment or push a new commit so we know to keep it open. Otherwise, this PR will be closed automatically in about one month. Thank you for your contribution to Apache Livy!

[idzikovsky]

This is still relevant by the way.

[github-actions]

This pull request has been automatically marked as stale because it has had no activity for at least 3 months. If you are still working on this change or plan to move it forward, please leave a comment or push a new commit so we know to keep it open. Otherwise, this PR will be closed automatically in about one month. Thank you for your contribution to Apache Livy!

[idzikovsky]

This is still relevant. Can I do something to help to proceed with this PR?