Add node cluster type

This adds the ability to create RAM cluster nodes (instead of only Disc nodes) by adding a `cluster_node_type`  parameter, and deprecates `cluster_disk_nodes` in favor of the (now) more accurate `cluster_nodes` parameter.

If defined, `cluster_disk_nodes` will be used and the user will be notified that the parameter is deprecated and that `cluster_nodes` should be used instead.

This pull request cleans up and replaces pull request #35