Allow default cluster_type

[dtjm]

This is a just a proof-of-concept. I welcome feedback on whether this is an idea you would accept, and if so, how to prepare the PR to your standards.

If cluster_type is not specified on the command line, and only 1 cluster_type is configured, then the command will use that cluster_type.

The purpose is to simplify the command-line invocation when you always have only 1 cluster type.

Thanks!