Fix lazy translation issue with watcher-db-manage

In this changeset, I fix the issue caused by the use of lazy
translations within the 'watcher-db-manage purge' subcommand.
This is caused by the PrettyTable dependency which performs
addition operations to format its tables and the __add__ magic
method is not supported by oslo_i18n._message.Message objects.

Change-Id: Idd590e882c697957cfaf1849c3d51b52797230f6
Closes-Bug: #1584652

watcher/_i18n.py

@@ -15,6 +15,7 @@
 # limitations under the License.
 #
 import oslo_i18n
+from oslo_i18n import _lazy
 
 # The domain is the name of the App which is used to generate the folder
 # containing the translation files (i.e. the .pot file and the various locales)
@@ -42,5 +43,9 @@ _LE = _translators.log_error
 _LC = _translators.log_critical
 
 
+def lazy_translation_enabled():
+    return _lazy.USE_LAZY
+
+
 def get_available_languages():
     return oslo_i18n.get_available_languages(DOMAIN)