[ZEPPELIN-1655] Dynamic forms in Python interpreter do not work

[agoodm]

What is this PR for?

After #1534 , Dynamic Forms were no longer working in the python interpreter. This is because the Py4jZeppelinContext constructor did not initialize the _displayhook which is always called on post-execute.

What type of PR is it?

Bug Fix

What is the Jira issue?

ZEPPELIN-1655

How should this be tested?

Run the following %python paragraph, being sure that Py4j is installed:

%python
a, b, c = (1, 2, 3)
z.select("Choose a letter", ([a,"a"], [b,"b"], [c,"c"] ))

Questions:

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

[agoodm]

Here you go @bzz !

[bzz]

Thank you for the prompt update @agoodm

CI failure on Selenium test profile seem un-related:

Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 129.268 sec - in org.apache.zeppelin.integration.ZeppelinIT

Results :

Failed tests: 
  AuthenticationIT.startUp:95 null

Tests run: 17, Failures: 1, Errors: 0, Skipped: 0

Looks gearat to me, merging to master as a hotfix, if there is no further discussion.