[OMEMO] Set access_model=open on PEP elements
OMEMO plugin should set
eu.siacs.conversations.axolotl.bundles PEP nodes to allow using OMEMO between non-subscribers (e.g. in multiuser groups).
devicelist node already exists and not configured with
pubsub#access_model, publishing to it with publish-options could return
precondition-not-met. Gajim should reconfigure this PEP node first.