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