Improve uffizzi cluster create UX

[gadkins]

$ uffizzi cluster create --name=[NAME] --kubeconfig=[KUBECONFIG] --manifest=[MANIFEST]

Improvements

• If --name is not provided, autogenerate a random two-word name, e.g. "happy-lion"
• If --kubeconfig=[KUBECONFIG] is not provided, check for existing ~/.kube/config. If it exists, merge an update to it. If it does not exist, create this file.
• If --manifest is not provided, return an error that this is a required input