This provides a new kind of "world" backend, one that runs etcd over an
SSH connection. This is useful for situations where you want to run an
etcd cluster somewhere for clients across the net, but where you don't
want to expose the ports publicly.
If SSH authentication is setup correctly (using public keys) this will
tunnel over SSH for etcd to connect.
This patch does not yet support deploys over SSH, but that should be
fixed in the future as the world code gets cleaned up more.
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