TODO - agnostic get credentials function for operator user for test speed up

get credential fixture for getting a password for a username (see below for the def of username)

----
At a later date @paulomach can help us further improve them such that we can speed up our tests with:

which has a USERNAME env var that is defined in our tox file

the use case for this fixture is if you have multiple tests relying on a credentials, you can speed up perfromance (if the scope is module)