Add Readme on certs folder and test_integration to notify about test-only certs and keys

[acfusg]

A readme file should be added to the certs folders in the repo to clarify that certs and keys are for testing purposes only. This also helps avoiding false positives on future assessments of the repo.

In addition, similar should be put on the test_integrations.py script for the section that shows an SSH private key in use, to clearly note that it is for testing purposes and need to be replaced by the user with a valid key.