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/local_gnocchi.conf.compute

@@ -0,0 +1,53 @@
+# Sample ``local.conf`` for compute node for Watcher development
+# NOTE: Copy this file to the root DevStack directory for it to work properly.
+
+[[local|localrc]]
+
+ADMIN_PASSWORD=nomoresecrete
+DATABASE_PASSWORD=stackdb
+RABBIT_PASSWORD=stackqueue
+SERVICE_PASSWORD=$ADMIN_PASSWORD
+SERVICE_TOKEN=azertytoken
+
+HOST_IP=192.168.42.2 # Change this to this compute node's IP address
+#HOST_IPV6=2001:db8::7
+FLAT_INTERFACE=eth0
+
+FIXED_RANGE=10.254.1.0/24 # Change this to whatever your network is
+NETWORK_GATEWAY=10.254.1.1 # Change this for your network
+
+MULTI_HOST=1
+
+SERVICE_HOST=192.168.42.1 # Change this to the IP of your controller node
+MYSQL_HOST=$SERVICE_HOST
+RABBIT_HOST=$SERVICE_HOST
+GLANCE_HOSTPORT=${SERVICE_HOST}:9292
+
+DATABASE_TYPE=mysql
+
+# Enable services (including neutron)
+ENABLED_SERVICES=n-cpu,n-api-meta,c-vol,q-agt,placement-client
+
+NOVA_VNC_ENABLED=True
+NOVNCPROXY_URL="http://$SERVICE_HOST:6080/vnc_auto.html"
+VNCSERVER_LISTEN=0.0.0.0
+VNCSERVER_PROXYCLIENT_ADDRESS=$HOST_IP # or HOST_IPV6
+
+NOVA_INSTANCES_PATH=/opt/stack/data/instances
+
+# Enable the Ceilometer plugin for the compute agent
+enable_plugin ceilometer https://opendev.org/openstack/ceilometer
+disable_service ceilometer-acentral,ceilometer-collector,ceilometer-api
+
+LOGFILE=$DEST/logs/stack.sh.log
+LOGDAYS=2
+
+[[post-config|$NOVA_CONF]]
+[DEFAULT]
+compute_monitors=cpu.virt_driver
+[notifications]
+# Enable both versioned and unversioned notifications. Watcher only
+# uses versioned notifications but ceilometer uses unversioned. We
+# can change this to just versioned when ceilometer handles versioned
+# notifications from nova: https://bugs.launchpad.net/ceilometer/+bug/1665449
+notification_format=both