Internationalization (i18n) - Enable French locale

Our project should now enable its internationalization.
This patchset add the french locale to the project but also
refactors the codebase to following the oslo_i18n recommendations.

DocImpact
Implements: blueprint support-translation

Change-Id: I0e4fbf05d16afb5e25bac78438c640f147c754b1

watcher/cmd/api.py

@@ -24,8 +24,8 @@ from wsgiref import simple_server
 from oslo_config import cfg
 from oslo_log import log as logging
 
+from watcher._i18n import _
 from watcher.api import app as api_app
-from watcher.common.i18n import _
 from watcher import service