new-cluster-enabled and SBR

[patriknw]

new-cluster-enabled=false doesn't work well with SBR down-all. It doesn't have to be the down-all SBR strategy but there are also other situations when SBR will resort to downing of all nodes and a new cluster must be formed.

SBR and Cluster Bootstrap are two independent processes so it's not easy to say that new-cluster-enabled should be false unless SBR downed all nodes before the bootstrap.

In Kubernetes it should be safe to use required-contact-point-nr > 1 instead of new-cluster-enabled=false. I haven't even seen any problems with required-contact-point-nr=1.

We should at least document this caveat.