Create new job for testing of translated manual

Currently, we build a few localized manuals in the post queue.
Add a new gate-{name}-tox-checklang job that uses tox for testing the
builds. This should be non-voting now.

This is done for all repos that have localized manuals:
openstack-manuals, operations-guide, api-site.

Change-Id: I86b9825905373cdb72b34669fffac83547270ca9
Merge "Create new job for testing of translated manual"