Do not force the creation of per-project ServiceAccount and Namespace

This is at least a regression for us.

We pre-create service accounts for Gitlab that we use for configuring the kubernetes integration. Projects can share namespaces and service accounts. But since 11.5, adding new projects fail because Gitlab tries to create per-project namespaces and service accounts. If we provide Gitlab with cluster-admin permissions, Gitlab creates these namespaces and service accounts, but these are too limited so we cannot deploy what we need to where we need it.

To work around this, we have to manually change the Gitlab-created serviceaccount to have the necessary permissions, which is quite a bit of unecessary hassle for us.

Allowing us to explicitly specify which serviceaccount to use and making the creation of a per-project namespace + serviceaccount would be much appreciated.