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 reconfigures 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.