Fix up multiple node launching so that it works

This will automatically assume one master and multiple workers if types
are not specified. If types are specified, they will be used to populate
the categories in the Ansible inventory.

@a8wright I know you're working on the readme.md so I'm giving that a wide berth. When you're done add the following explanation in an appropriate place

For the ansible inventory file,
* if no node types are defined, we will assume that you want to create one master and n-1 worker nodes with the corresponding categories
* you can specify node types by adding the following to your cluster configurations, the following specifies two worker nodes, one database server, and one master

        [cluster8]
        number_of_nodes=4
        types=master::worker::worker::dbserver
        target_directory=target-aws-8