plugin pubsub cache: resync in `synchronise` when node's `sync_state` is not set: synchronisation used to be skipped when the node was existing and `resync` was not set, however the case happens when a node is subscribed but not synchronised, thus this patch update the behaviour to be sure to have the node synchronised. rel 369

these certificates are used for demo purpose only. Those must be the same as in
backend_e2e (they are generated there)