1605 get updated credentials

[mssalvatore]

What does this PR do?

Enables the AutomatedMaster to periodically check for new propagation credentials during the exploitation phase of the attack.

Issue #1605

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?

Testing Checklist

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

  Tested by running unit test
  Tested by running with modified source code to verify the behavior of the ControlClient

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

[ilija-lazoroski]

Nice explanation. I think we will just need to revise CREDENTIALS_POLL_PERIOD_SEC later on.