Improvements for uffizzi cluster update-kubeconfig

[gadkins]

Improvements

• If you specify a path with the --kubeconfig option, then the resulting configuration file is created there or merged with an existing kubeconfig at that location.
• Add option -p / --print to dump kubeconfig to stdout, e.g. uffizzi cluster update-kubeconfig -p
• Add option -q / --quiet to suppress output
• If you have the KUBECONFIG environment variable set, then the resulting configuration file is created at the first entry in that variable or merged with an existing kubeconfig at that location.
• If a previous cluster configuration exists for a Uffizzi cluster with the same name at the specified path, the existing configuration is overwritten with the new configuration.
• When update-kubeconfig writes a configuration to a kubeconfig file, the current-context of the kubeconfig file is set to that configuration.