Set --kubeconfig flag as optional in k3kcli cluster <cmd>

[enrichman]

The k3kcli cluster commands need the --kubeconfig flag.

This should not be mandatory, and it should follow the same logic of kubectl (see doc):

• If the --kubeconfig flag is set, then only that file is loaded. The flag may only be set once and no merging takes place.

• If $KUBECONFIG environment variable is set, then it is used as a list of paths (normal path delimiting rules for your system). These paths are merged. When a value is modified, it is modified in the file that defines the stanza. When a value is created, it is created in the first file that exists. If no files in the chain exist, then it creates the last file in the list.

• Otherwise, ${HOME}/.kube/config is used and no merging takes place.

It should work by default, but if not I think we can avoid the merging of multiple files, if complicated, and just add the fallback to the default location ${HOME}/.kube/config.