Make prometheus the default devstack example

Change the devstack local.conf samples and devstack multinode
contributor doc to demonstrate deploying watcher with prometheus as
datasource instead of gnocchi. Keep the gnocchi as an alternative
deployment example.

Depends-On: https://review.opendev.org/c/openstack/watcher/+/946230
Depends-On: https://review.opendev.org/c/openstack/devstack-plugin-prometheus/+/946254

Change-Id: I721b550a03f9e5350a3f1ab10292faa1c50049a7

devstack/prometheus.yml

@@ -0,0 +1,16 @@
+global:
+  scrape_interval: 10s
+scrape_configs:
+  - job_name: "node"
+    static_configs:
+      - targets: ["controller:3000"]
+      - targets: ["controller:9100"]
+        labels:
+          fqdn: "controller" # change the hostname here to your controller hostname
+      - targets: ["compute-1:9100"]
+        labels:
+          fqdn: "compute-1" # change the hostname here to your fist compute hostname
+      - targets: ["compute-2:9100"]
+        labels:
+          fqdn: "compute-2" # change the hostname her to your secondd compute hostname
+      # add as many blocks as compute nodes you have