[ZEPPELIN-4580] check the instance type rather than the class name

[jeff-hammel-leapyear]

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

This checks if the realm is an instance of KnoxJwtRealm instead of checking for it by name to enable subclassing of KnoxJwtRealm.

What type of PR is it?

[Bug Fix]

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]

https://issues.apache.org/jira/browse/ZEPPELIN-4580

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.

CI run is at https://travis-ci.org/LeapYear/zeppelin-source/builds/650518752

Screenshots (if appropriate)

Questions:

• Does the licenses files need update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[jeff-hammel-leapyear]

This is a port of #3618 vs master instead of the 0.8 branch. I will post CI results and put up for review with Travis finishes

[jeff-hammel-leapyear]

CI run is at https://travis-ci.org/LeapYear/zeppelin-source/builds/650518752

I tried to go through and figure out the failures. Those that I could seemed to be unrelated to my change. I also compared to the run for the previous commit on master: https://travis-ci.org/LeapYear/zeppelin-source/builds/650515988 . Both had errors, albeit not an identical set. The job output is large and on top of being hard to read doesn't fully load in my browser (Chrome). If there is anything specific I should look for, please point me to the right place.

[zjffdu]

Thanks @jeff-hammel-leapyear The failed test is unrelated. I will merge your PR soon if no more comment