Set access_model=open on OMEMO PEP elements. Fixes #444.

OMEMO plugin should set pubsub#access_model to open for eu.siacs.conversations.axolotl.devicelist and eu.siacs.conversations.axolotl.bundles PEP nodes to allow using OMEMO between non-subscribers (e.g. in multiuser groups).

If 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.