Reflect KubernetesService template to cluster instances immediately
Description
On the current implementation, it takes time to reflect a change of KubernetesService template to cluster instances.
According to @bikebilly, this behavior should be the following.
- if you enable the service template, a new cluster is provided to any project that doesn't have a cluster yet
- the cluster will be visible in CI/CD > Kubernetes page
- if you disable the service template, that cluster is not available or visible anymore to any project
https://gitlab.com/gitlab-org/gitlab-ce/issues/39217#note_75684697