kube-prometheus/thanos: Move to experimental folder

In preparation of PR #1206 [1], This patch moves the Thanos manifests to the
`experimental` folder.

[1] https://github.com/coreos/prometheus-operator/pull/1206

experimental/thanos/thanos-peers-svc.yaml

@@ -0,0 +1,14 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: thanos-peers
+spec:
+  type: ClusterIP
+  clusterIP: None
+  ports:
+  - name: cluster
+    port: 10900
+    targetPort: cluster
+  selector:
+    # Useful endpoint for gathering all thanos components for common gossip cluster.
+    thanos-peer: "true"