Validate cluster UUID values

Ensure that any cluster UUID values which can be specified by a human or operator are conformant to the UUID spec. Otherwise, follow-up operations may start to fail in hard-to-debug ways.

The motivation here was a case where a cluster UUID was copied from the cloud control panel with the a query parameter like `?=` appended. This would short-circuit HTTP requests to the DO API carried out by godo and not return sub-resources anymore.