Simplify conditionals/DRY up the k3d dev script

Main change in this MR is an incremental build of the k3d cluster create command. This saves us from a mess of nested conditionals and handling every combination of options. It prevents a lot of repetition in the commands used and makes it easier to extend with other k3d options if needed.

Assuming I changed things properly this shouldn't change any behavior, just the under the hood code.