Auto-detect if Prometheus has already been deployed to a Cluster

Today, you can connect a single cluster to multiple projects. Unfortunately, you cannot install Prometheus multiple times, as we deploy to a single namespace.

If a cluster already has Prometheus, we should simply auto-detect it's presence and enable the service. This will be particularly important for group-level clusters (https://gitlab.com/gitlab-org/gitlab-ce/issues/34758), once they arrive in %10.8.