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/common/utils.py

@@ -24,7 +24,7 @@ import six
 import uuid
 
 
-from watcher.common.i18n import _LW
+from watcher._i18n import _LW
 
 UTILS_OPTS = [
     cfg.StrOpt('rootwrap_config',