HDDS-3332. Upgrade Robot tests to Python 3

[adoroszlai]

What changes were proposed in this pull request?

Upgrade Robot tests to Python 3 by using a newer ozone-runner image, which has Python 3 installed.

https://issues.apache.org/jira/browse/HDDS-3332

How was this patch tested?

CI: https://github.com/adoroszlai/hadoop-ozone/runs/558100142

Tested that runaway command is stopped after the specified timeout (10 seconds):

$ time docker run -it --rm -v $(pwd):/tmp apache/ozone-runner:20200403-1 robot /tmp/tmp.robot
To use Ozone please mount ozone folder to /opt/hadoop
==============================================================================
Tmp :: Timeout test
==============================================================================
Test timeout behavior with wait                                       | FAIL |
Test timeout 10 seconds exceeded.
------------------------------------------------------------------------------
Tmp :: Timeout test                                                   | FAIL |
1 critical test, 0 passed, 1 failed
1 test total, 0 passed, 1 failed
==============================================================================
Output:  /opt/hadoop/output.xml
Log:     /opt/hadoop/log.html
Report:  /opt/hadoop/report.html
docker run -it --rm -v $(pwd):/tmp apache/ozone-runner:20200403-1 robot   0.03s user 0.09s system 1% cpu 11.384 total

whereas previously it ran until completion (30 seconds):

$ time docker run -it --rm -v $(pwd):/tmp apache/ozone-runner:20191107-1 robot /tmp/tmp.robot
To use Ozone please mount ozone folder to /opt/hadoop
==============================================================================
Tmp :: Timeout test
==============================================================================
Test timeout behavior with wait                                       | FAIL |
Test timeout 10 seconds exceeded.
------------------------------------------------------------------------------
Tmp :: Timeout test                                                   | FAIL |
1 critical test, 0 passed, 1 failed
1 test total, 0 passed, 1 failed
==============================================================================
Output:  /opt/hadoop/output.xml
Log:     /opt/hadoop/log.html
Report:  /opt/hadoop/report.html
docker run -it --rm -v $(pwd):/tmp apache/ozone-runner:20191107-1 robot   0.03s user 0.07s system 0% cpu 31.267 total

[elek]

Thanks to put this final piece in. I am +1 if CI agrees...

[dineshchitlangia]

+1 Thanks @adoroszlai for the improvement and @elek for the reviews.

[adoroszlai]

Thanks @elek and @dineshchitlangia for the review.