Add architecture to the binaries and images in the cache

[afbjorklund]

When supporting remote clusters of a different architecture, we need to add the arch in addition to the OS:

~/.minikube/cache/linux/v1.19.3
├── kubeadm
├── kubectl
└── kubelet

Simplest would be to add it after the OS, but we could support both in order to preserve existing user caches ?

~/.minikube/cache/linux
├── amd64
│   └── v1.19.3
│       ├── kubeadm
│       ├── kubectl
│       └── kubelet
└── arm64
    └── v1.19.3
        ├── kubeadm
        ├── kubectl
        └── kubelet

For instance when running an amd64 host, towards a arm64 machine. Either local hardware or cloud instance.

• Raspberry Pi

• AWS Graviton

Same thing for the images, if not using the preload (the preloaded-tarball files already include the arch, though)

~/.minikube/cache/images
├── gcr.io
│   └── k8s-minikube
│       └── storage-provisioner_v3
├── k8s.gcr.io
│   ├── coredns_1.7.0
│   ├── etcd_3.4.13-0
│   ├── kube-apiserver_v1.19.2
│   ├── kube-controller-manager_v1.19.2
│   ├── kube-proxy_v1.19.2
│   ├── kube-scheduler_v1.19.2
│   └── pause_3.2
└── kubernetesui
    ├── dashboard_v2.0.3
    └── metrics-scraper_v1.0.4

~/.minikube/cache/images/amd64
~/.minikube/cache/images/arm64

[afbjorklund]

Also, this code needs to be parameterized:

                        src, err := download.Binary(name, cfg.KubernetesVersion, "linux", runtime.GOARCH)
                        if err != nil {
                                return errors.Wrapf(err, "downloading %s", name)
                        }

                       if _, err := download.Binary(bin, version, "linux", runtime.GOARCH); err != nil {
                                return errors.Wrapf(err, "caching binary %s", bin)
                        }

func archTag(hasTag bool) string {
        if runtime.GOARCH == "amd64" && !hasTag {
                return ":"
        }
        return "-" + runtime.GOARCH + ":"
}

var defaultPlatform = v1.Platform{
        Architecture: runtime.GOARCH,
        OS:           "linux",
}

Basically split host arch and cluster (node?) arch.

Using runtime.GOARCH is a good default, but it needs to be possible to override.

It is still OK to hardcode the OS (for now...), since Kubernetes only runs on Linux.

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