ZEPPELIN-3792. Support Kerberos Realm

[zjffdu]

What is this PR for?

A few sentences describing the overall goals of the pull request's commits.
First time? Check out the contributing guide - https://zeppelin.apache.org/contribution/contributions.html

What type of PR is it?

[Bug Fix | Improvement | Feature | Documentation | Hot Fix | Refactoring]

Todos

• - Task

What is the Jira issue?

• Open an issue on Jira https://issues.apache.org/jira/browse/ZEPPELIN/
• Put link here, and add [ZEPPELIN-Jira number] in PR title, eg. [ZEPPELIN-533]

How should this be tested?

• First time? Setup Travis CI as described on https://zeppelin.apache.org/contribution/contributions.html#continuous-integration
• Strongly recommended: add automated unit tests for any new or changed behavior
• Outline any manual steps to test the PR here.

Screenshots (if appropriate)

Questions:

• Does the licenses files need update?
• Is there breaking changes for older versions?
• Does this needs documentation?

[Tagar]

Thanks @zjffdu !

Would be nice to have documentation as a reference on how to setup Kerberos realm for Zeppelin authentication. Or at least documentation that it's supported with perhaps links to Shiro with examples of shiro.ini.