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.controller

@@ -0,0 +1,57 @@
+# Sample ``local.conf`` for controller 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=$ADMIN_PASSWORD
+SERVICE_TOKEN=azertytoken
+
+HOST_IP=192.168.42.1 # Change this to your controller node IP address
+#HOST_IPV6=2001:db8::7
+FLAT_INTERFACE=eth0
+
+FIXED_RANGE=10.254.1.0/24 # Change this to whatever your network is
+NETWORK_GATEWAY=10.254.1.1 # Change this for your network
+
+MULTI_HOST=1
+
+
+#Set this to FALSE if do not want to run watcher-api behind mod-wsgi
+#WATCHER_USE_MOD_WSGI=TRUE
+
+# This is the controller node, so disable nova-compute
+disable_service n-cpu
+
+# Enable the Watcher Dashboard plugin
+enable_plugin watcher-dashboard https://opendev.org/openstack/watcher-dashboard
+
+# Enable the Watcher plugin
+enable_plugin watcher https://opendev.org/openstack/watcher
+
+# Enable the Ceilometer plugin
+enable_plugin ceilometer https://opendev.org/openstack/ceilometer
+
+# This is the controller node, so disable the ceilometer compute agent
+disable_service ceilometer-acompute
+
+# Enable the ceilometer api explicitly(bug:1667678)
+enable_service ceilometer-api
+
+# Enable the Gnocchi plugin
+enable_plugin gnocchi https://github.com/gnocchixyz/gnocchi
+
+LOGFILE=$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