Refs #4478 - Add support for localized API docs

This is Katello part of localization of API docs. The API doc strings serve as a source  for API documenatation in HTML and description of the API in JSON which is used in CLI.
With this patch all the API doc strings are wrapped in _() and enabled for translation.

Foreman part is here https://github.com/theforeman/foreman/pull/1428 and related apipie-rails changes are here https://github.com/Apipie/apipie-rails/pull/232