Fix issues on variable names and API calls

• Update the TF_VAR_username to be the user name, not Kristof hardcoded.
• Format TF files.
• Echo GOOGLE_CREDENTIALS, since they are in a variable, not a file.
• Call gcloud projects add-iam-policy-binding for each role since the API does not allow calling it with multiple roles.
• Add documentation on GCP roles usage.
• Improve doc on PAT usage.
• Update TF variables to match GCP wording for ease of usage.