Create and use service account with Terraform

[jgraeger]

Right now we are using a predefined GCP service account for all tasks performed by terraform.
We want to create the required service accounts also via terraform using only a restricted predefined service account with IAM roles.

In the current state of the infrastructure, the used predefined services account requires the GCP "Editor" and "Cluster-Administrator" roles.

Follow-Up to #10.

[jgraeger]

Due to adding google_project_service in #34, also the Service Usage-Administrator role is required.