Python interpreter and doc cleanup

[bzz]

What is this PR for?

This is first step improving current Python interpreter implementation.
It has just a cleanup, style and docs improvements.

What type of PR is it?

Improvement

Questions:

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

[bzz]

CI failure is not related and is taken care of under #1015

Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 34.3 sec - in org.apache.zeppelin.integration.SparkParagraphIT
Results :
Tests in error: ParagraphActionsIT.testRemoveButton:153 » NoSuchElement Unable to locate eleme...
ParagraphActionsIT.testCreateNewButton:86 » NoSuchElement Unable to locate ele...
Tests run: 14, Failures: 0, Errors: 2, Skipped: 0

Ready to be merged to master now.

[felixcheung]

LGTM

[AhyoungRyu]

LGTM 👍

[astroshim]

LGTM

[prabhjyotsingh]

LGTM

[bzz]

Thank you for prompt reviews!

Merging if there is no more discussion.