plugin XEP-0384, XEP-0391: download missing devices list: when a peer jid was not in our roster, devices list was not retrieved, resulting in failed en/decryption. This patch does check it and download missing devices list in necessary. There is no subscription managed yet, so the list won't be updated in case of new devices, this should be addressed at some point.

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)