PropagationConfiguration docstring + validation

[shreyamalviya]

What does this PR do?

Fixes a part of #2004

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  UTs pass

• If applicable, add screenshots or log transcripts of the feature working
  

[codecov]

Codecov Report

Merging #2129 (6da238f) into develop (643896b) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 6da238f differs from pull request most recent head c2052a5. Consider uploading reports for the commit c2052a5 to get more accurate results

@@           Coverage Diff            @@
##           develop    #2129   +/-   ##
========================================
  Coverage    56.64%   56.64%           
========================================
  Files          486      486           
  Lines        13227    13227           
========================================
  Hits          7493     7493           
  Misses        5734     5734           

Impacted Files	Coverage Δ
...t_configuration/agent_sub_configuration_schemas.py	100.00% <100.00%> (ø)
...on/agent_configuration/agent_sub_configurations.py	100.00% <100.00%> (ø)

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