lxd/cluster/events: Updates EventsUpdateListeners to only connect to event-hub servers
lxd/cluster/events: Updates EventsUpdateListeners to only connect to event-hub servers

If there are at least 2 event-hub servers.
Otherwise we continue to operate in full-mesh mode to ensure we don't miss events if a single event-hub member goes down.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd