[AIRFLOW-840] Make ticket renewer python3 compatible

[abij]

The return from the subprocess is in bytes when the universal
newlines is set to False (default). This will fail in Py3 and
works fine in Py2.

Please accept this PR that addresses the following issues:

• https://issues.apache.org/jira/browse/AIRFLOW-840

[mention-bot]

@abij, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bolkedebruin, @jlowin and @skudriashev to be potential reviewers.

[bolkedebruin]

Let us discuss this when I'm back. You probably need to add an extra User. I think it is a permission problem

[codecov-io]

Codecov Report

Merging #2158 into master will increase coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2158      +/-   ##
==========================================
+ Coverage   67.24%   67.48%   +0.24%     
==========================================
  Files         142      142              
  Lines       10823    10824       +1     
==========================================
+ Hits         7278     7305      +27     
+ Misses       3545     3519      -26

Impacted Files	Coverage Δ
airflow/security/kerberos.py	72.09% <100%> (+72.09%)	⬆️
airflow/jobs.py	72.94% <0%> (-0.42%)	⬇️

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[bolkedebruin]

Looks good in general. Are all changes required to setup_kdc.sh? Please also squash your commits into one.

[abij]

Squeezed into single commit and removed some unnecessary bash commands in setup_kdc.sh

[bolkedebruin]

Lgtm!