By definition, `single-node` clusters can only consist of 1 node. Likewise,
`multi-node` clusters must have 2 or more nodes.
* Ensure `ClusterType` parameter is either `multi-node` or `single-node`.
* Ensure proper validation of `NumberOfNodes` parameter based on `ClusterType`
parameter.
* Fix existing test case that incorrectly allowed a `multi-node` cluster to
consist of 1 node.
* Add dedicated test for resizing a cluster from `single-node` to `multi-node`
and back again.
Behavior and error messages have been verified against a real AWS backend.
dcd034045b