We separate out the `create cluster` operation from the `update cluster`
operation. Now create cluster only creates the spec (unless you pass
--yes), and is only for new clusters.
`update cluster` works on new or existing clusters, and should be called
to apply changes.
`update` is not the best name, because it means something different in
kubectl, but I think it's a good start.
We choose the longest matching existing hosted zone as the default,
rather than the previous heuristic which was overly simplistic.
We also require the hosted zone to exist now; it doesn't seem to really
cost us anything given that the user has to set up DNS delegation
anyway.
Fix#125
Users can still get HA master by explicitly specifying a list of
`--master-zones`.
But HA master is not as well tested, is slower, needs more machines etc
and we probably shouldn't silently force it as the default.
Fix#33