Add credentials folder and .gitignore, and move secrets to that folder

[stea-uw]

I moved the secrets in this file to a yaml file in the credentials/ directory.

Tested the config locally by running test_scan.py with both *conf.py files.

[stea-uw]

How to deploy this change:

1. Create a "db.yaml" file with the credentials (you can copy it from idp11) and put it in "credentials/"
2. Copy over the iamtools-*.pem files into the "credentials/" directory as well
3. Copy over the "local-bin/" directory from the latest main into the "local-bin/" on the server
4. Run "test_scan.py" to test that it worked. E.g:

   . py-env/bin/activate
   export PATH=$PATH:/usr/local/pgversion/bin
   python test_scan.py
   

I already did 1) and 2) for the prod machines, but didn't have access to the test machines for some reason.

See also credentials/README.md