Create kubectl bash-completion script

[NicolasT]

The kubectl package we install on some nodes doesn't come with a bash-completion completion script (unlike the kubernetes-client Fedora package). When bash-completion is installed on a system, getting kubectl completion is useful though.

We could do this as part of the Salt state by creating /etc/bash_completion.d and putting the output of kubectl completion bash in file called kubectl in said directory (cfr. 05b4be6). Keep in mind, however, we want this state (which likely needs cmd.run or similar) to be idempotent, i.e., render the completions into a variable or some temporary file, then update /etc/bash_completion.d/kubectl only when the completion script has changed.