FactoryGirl traits `cluster_provider_gcp` and `cluster_platform_kubernetes` creates meaningless cluster records

For example, we create this data for tests.

let(:cluster) { create(:cluster, :provided_by_gcp, projects: [project]) }

This creates three clusters, because provider_gcp and platform_kubernetes has a cluster definition in factory.

But those clusters don't point to the given cluster, so two orphans always exist.

Ideally, this should be always one creation.

/cc @ayufan @grzesiek @matteeyah