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/api/controllers/v1/utils.py

@@ -17,7 +17,7 @@ import jsonpatch
 from oslo_config import cfg
 import wsme
 
-from watcher.common.i18n import _
+from watcher._i18n import _
 
 CONF = cfg.CONF