Enable prometheus and node_exporter from devstack-plugin-prometheus

https://opendev.org/openstack/devstack-plugin-prometheus is the new
devstack plugin providing functionality to install/configure
prometheus/node_exporter.

It will replace sg_core devstack plugin in future.

Depends-On: https://review.opendev.org/c/openstack/watcher/+/938893
Depends-On: https://review.opendev.org/c/openstack/devstack-plugin-prometheus/+/940426

Change-Id: Ia75e6597275b36c04cde653c16f7d45ed23bc261
Signed-off-by: Chandan Kumar (raukadah) <chkumar@redhat.com>

playbooks/generate_prometheus_config.yml

@@ -0,0 +1,9 @@
+---
+- hosts: all
+  tasks:
+    - name: Generate prometheus.yml config file
+      delegate_to: controller
+      template:
+        src: "templates/prometheus.yml.j2"
+        dest: "/home/zuul/prometheus.yml"
+        mode: "0644"