Handle translations for api-site

The generatedocbook command only handled books in subdirectories of
./doc. This assumption only works for operations-guide and
openstack-manuals but not for the api-site. Add parameter --root to give
the root directory. For the api-site, this would be "--root ./".

Change-Id: I9f477239677ca5a32b19bd565f8a37288f7e869e
Merge "Handle translations for api-site"