Add a shared user for api credentials

Add an api_user key to the credentials file
Remove logic for non-Persona login
Remove unused username key from credentials file

This pull is to support having a shared user for accessing the api. All tests will need to use a user with an api key and this will simplify new contributors coming on. Also have removed the logic for logging into a non-Persona-enabled version of Moztrap as we do not test such an environment.

Note that there will be an associated webqa-credentials pull request to go with this.