Management project: Disable creation of project namespace and project service account

Problem

If we were using a managed cluster this would create a production namespace and service account, which wouldn't actually be used for anything. We use cluster-admin tokens for management projects (see https://gitlab.com/gitlab-org/gitlab/blob/0cb9015f20c7705c6c00f44856cbf5eeac25d229/app/models/clusters/platforms/kubernetes.rb#L76).

Proposal

Disable creation of project namespace and project service account for jobs where management project is used.