By setting KUBE_SHARE_MASTER=true we reuse an existing master, rather
than creating a new one.
By setting KUBE_SUBNET_CIDR=172.20.1.0/24 you can specify the CIDR for a
new subnet, avoiding conflicts.
Both these options are documented only in kube-up and clearly marked as
'experimental' i.e. likely to change.
By combining these, you can kube-up a cluster normally, and then kube-up
a cluster in a different AZ, and the new nodes will attach to the same
master.
KUBE_SHARE_MASTER is also useful for addding a second node
auto-scaling-group, for example if you wanted to mix spot & on-demand
instances.
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