Move Gcp::Kubernetes* services outside GCP namespace

We have 3 classes under Clusters::Gcp::Kubernetes:

• Clusters::Gcp::Kubernetes::CreateOrUpdateNamespaceService (after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22011)
• Clusters::Gcp::Kubernetes::CreateServiceAccountService
• Clusters::Gcp::Kubernetes::FetchKubernetesTokenService

But this services should apply for all Kubernetes clusters, not just the ones for GCP. We should move them to a different namespace/folder, maybe Clusters::Kubernetes?