util.pubsub: For open nodes, default affiliation is "member"
This allows entities without an explicit affiliation to retrieve items,
which is specified by the XEP. Table 6: "Node Access Models" states that
for 'open' nodes, "any entity may retrieve items from the node".
See also discussion at:
https://mail.jabber.org/pipermail/standards/2018-August/035320.html