multi-node: unable to choose CNI (always applies kindnet)

[sharifelgamal]

Right now we enforce our own CNI for multinode clusters, ignoring any input from the user. We should respect the --enable-default-cni parameter and not apply CNI when that's false.

[tstromberg]

The TL;DR is that CNI in minikube is very confusing.

• There is --enable-default-cni (default: false), which enables the CNI plug-in, but does not setup CNI.
• There is --network-plugin=, required for CNI. The only option we currently support is cni, though there are some other more obscure alternatives in Kubernetes: https://kubernetes.io/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/

Here is my rough proposal, which would also help address #1568

• Change --enable-default-cni (bool) flag to --cni (string).

  • none: no CNI. The current default for minikube.
  • kindnet: The current default for minikube if -n or --driver=docker is used.
  • flannel: kindnet alternative.
  • custom: bring your own CNI (for backwards compat, will also respect true).

• Add alias for --enable-default-cni to --cni.

If things look good, I also feel that --cni=kindnet should be the default for all clusters.