fix TypeError in python3

[SathishMSFT]

Bug: https://msazure.visualstudio.com/One/_workitems/edit/17037023

Issue: In python3, b64encode return byte string. This needs to be decoded to a normal string before parsing with json. else it returns TypeError. Issue found as of now only in sles12 and ubuntu16 when python 3 versions are 3.4 and 3.5.2 respectively. The ubuntu 18 VM where it was working has python version 3.8.2.

Test done:

• UT for python2 and python3
• Manually tested in ubuntu 18, rhel8, suse12.
• SLES15, ubuntu16 images not available in Azure. For ubuntu 16, repro-ed the issue locally and verified the fix.

[codecov]

Codecov Report

Merging #168 (fc8e136) into master (64c79c6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #168   +/-   ##
=======================================
  Coverage   88.51%   88.52%           
=======================================
  Files          86       86           
  Lines       12595    12598    +3     
=======================================
+ Hits        11149    11152    +3     
  Misses       1446     1446           

Flag	Coverage Δ
python27	86.85% <100.00%> (+<0.01%)	⬆️
python39	88.49% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core/src/bootstrap/Bootstrapper.py	75.65% <100.00%> (+0.43%)	⬆️
src/core/src/core_logic/ExecutionConfig.py	81.51% <100.00%> (+0.15%)	⬆️

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