Warn when running a one-node cluster
If clustering is enabled, but only one node is running, we should issue warnings.
Although every cluster node will briefly be a one-node cluster during start-up ( warnings should not be sent then), having a one node cluster implies that the expectation of having cluster benefits (redundancy, performance) are not met.
display a warning in the admin console, whenever the cluster is just one node
send out a message to all admins (akin to the messages send when plugin updates are available), when the cluster shrinks from multiple, to one node)
send out a recovery message to all admins, but only if a warning message was sent earlier (no need to 'recover' from a cluster that is simply starting up).