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.