[ZEPPELIN-5999] Reduce instance objects from Zeppelin

[Reamer]

What is this PR for?

This PR removes the most important instance objects from Zeppelin. This PR standardizes our integration tests via the ZeppelinServerMini class. I was previously unaware that the Zeppelin configuration is also used within interpreters. With these changes I noticed it and the Zeppelin configuration is now available to interpreters. I have tried to split the PR into meaningful commits. Unfortunately, it is not possible to split the PR further due to the mass of uses of the instance objects.

The ZeppelinServerMini class makes it easy to run integration tests in the IDE. Separate steps such as "download Spark", "download Hadoop", "add SPARK_HOME env" or "start Zeppelin" become obsolete.

You can now also see quite clearly which tests are actually simple unit tests and which should actually be executed as integration tests. A switch to integration tests can be made in a separate pull request.

My IDE and locale are set to German. Java adopts this and also outputs some exceptions in German. I don't know how we deal with this, so I have removed language specific asserts.

      // depends on JVM language
      // assertTrue(stacktrace.contains("No such file or directory"), stacktrace);

What type of PR is it?

• Improvement

Todos

• [ ] - Task

What is the Jira issue?

• https://issues.apache.org/jira/browse/ZEPPELIN-5999

How should this be tested?

• CI

Questions:

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

[Reamer]

I know that the PR is very big. Does anyone feel able to do a review here?

[Reamer]

Ready for review.

[Reamer]

Thank you for your review. My plan is to merge #4746 first and then do this MR.

[jongyoul]

LGTM. Please feel free to merge it when you're available.