This commit fixes the ejabberd tests for hydra:

mod_http_upload and mod_disco need to be explicitly enabled, and a
handler needs to be setup to make it work. Also, the client needs to be
able to contact the server.

The commit also fixes the situation where http upload failed: in that
case the client would wait forever because nothing catched the error.

Finally, there remains a non-reproducible error where ejabberd server
fails to start with an error like:
format: "Failed to create cookie file '/var/lib/ejabberd/.erlang.cookie': eacces"
(happens ~15%) I tried to check existence of /var/lib/ejabberd/ in
pre-start script and saw nothing that would explain this error, so I
gave up about this error in particular.

(cherry picked from commit cdaec7e)