Add rke command completion

[weh]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
• The code is mine or it's from somewhere with an MIT-compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.

Changes:

• Adds the Rancher Kubernetes Engine rke CLI completion

[weh]

@mcornella do you want completions like this? i plan to add k3s also, since i use it regularly.