Add new env for tox to test doc publication in the FTP

The script `upload_doc.sh` has been updated to allow passing a destination
folder where to upload the content to the FTP.

A new environment `test-publish`has been added to tox.ini to testing doc
publication, making use of the updates in `upload_doc.sh`

Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>