plugin XEP-0373: download public keys if they are not found in local storage: public keys were only obtained from PEP notifications, however this wasn't working if the entity was not in our roster. Now if no public key is retrieved from local storage, the public key node is requested, and an error is raised if nothing is found. This allows the use of OX with entities which are not in roster. rel 380

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