Updated the config sample file generation command

Following the update of the project requirements, we should now
use the oslo.config namespace of oslo.log when generating ou
Watcher config sample file.

Change-Id: I7a1f3d555534b40c041b7f042f19fae231b7e80c
Closes-Bug: #1518297

tox.ini

@@ -32,6 +32,7 @@ sitepackages = False
 commands =
   oslo-config-generator --namespace watcher \
                         --namespace keystonemiddleware.auth_token \
+                        --namespace oslo.log \
                         --namespace oslo.db \
                         --output-file etc/watcher/watcher.conf.sample