Removed 'watcher_messaging' to use oslo.messaging

The old 'watcher_messaging' section of the Watcher configuration file
has now been replaced by the more standard oslo.configuration one.

DocImpact

Change-Id: Ie027df023e6133f3188e57b42846083f28c282bd

tox.ini

@@ -34,6 +34,7 @@ commands =
                         --namespace keystonemiddleware.auth_token \
                         --namespace oslo.log \
                         --namespace oslo.db \
+                        --namespace oslo.messaging \
                         --output-file etc/watcher/watcher.conf.sample
 
 [flake8]