Fixes #4478 - Added API localization

This PR contains 
- localization setup for apipie
- rake task for building documentation in parts (index pages and resources documentation)

Doc build duration for reference:  Foreman in sec (Foreman + Katello in min:sec) for all supported languages
```pre
rake apipie:cache cache_part=resources:      3:00 (5:44)
rake apipie:cache cache_part=index           0:54 (1:48)
rake apipie:cache                            3:24 (6:39)
```
Resources can be built during package build time and index after package installation.

Known issues:
- when installed with katello plugin the index cache rake in posttrans fails silently because katello is not configured (should work after katello-installer is run -> useful for update)
- if only indexes are rebuild during rpm install or in foreman-installer all plugins providing API should have the resource docs packaged. If they have not, Foreman API documentation shows '404', but API bindings (Hammer) should not be affected as it use the index only.

Related PR in apipie-rails https://github.com/Apipie/apipie-rails/pull/232
Related PR with API doc string marked for translation https://github.com/theforeman/foreman/pull/1626