Set access_model=open on OMEMO PEP elements. Fixes #444.
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. The plugin will try to reconfigure this PEP node.
NOTE: this modification does not check the presence of pubsub#config-node and pubsub#create-and-configure features and tries to use it anyway.
NOTE 2: reconfiguration uses Data Forms protocol but does not request a Form first, and just tries to reconfigure a single form element.