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