Add important requirements to none driver page

[afbjorklund]

We now link to the kubeadm documentation directly from https://minikube.sigs.k8s.io/docs/drivers/none/, which is good. But we should list some of the more important requirements from kubeadm explicitly:

• 2 CPU

• 2200M RAM available

• iptables (in legacy mode)

• conntrack (and others?)

• SELinux permissive

• Downgrade to cgroups v1

Maybe we should rephrase "VM" as something virtual or physical:

A Linux VM with the following:

Docker
systemd (OpenRC based systems are also supported in v1.10+)

This should probably say "container runtime" instead of just Docker. Also need to install crictl

At least when we fix the bug that all runtimes currently require a docker binary in order to start

---

Most of these should probably be validated, and give helpful messages.

This to cut down on the number of identical support issues being opened.

[afbjorklund]

Here is the current output, with --driver=none:

! initialization failed, will try again: run: /bin/bash -c "sudo env PATH=/var/lib/minikube/binaries/v1.18.0:$PATH kubeadm init --config /var/tmp/minikube/kubeadm.yaml  --ignore-preflight-errors=DirAvailable--etc-kubernetes-manifests,DirAvailable--var-lib-minikube,DirAvailable--var-lib-minikube-etcd,FileAvailable--etc-kubernetes-manifests-kube-scheduler.yaml,FileAvailable--etc-kubernetes-manifests-kube-apiserver.yaml,FileAvailable--etc-kubernetes-manifests-kube-controller-manager.yaml,FileAvailable--etc-kubernetes-manifests-etcd.yaml,Port-10250,Swap,SystemVerification": exit status 1
stdout:
[init] Using Kubernetes version: v1.18.0
[preflight] Running pre-flight checks

stderr:
W0502 08:40:45.681001    4513 configset.go:202] WARNING: kubeadm cannot validate component configs for API groups [kubelet.config.k8s.io kubeproxy.config.k8s.io]
	[WARNING IsDockerSystemdCheck]: detected "cgroupfs" as the Docker cgroup driver. The recommended driver is "systemd". Please follow the guide at https://kubernetes.io/docs/setup/cri/
	[WARNING Swap]: running with swap on is not supported. Please disable swap
	[WARNING Service-Kubelet]: kubelet service is not enabled, please run 'systemctl enable kubelet.service'
error execution phase preflight: [preflight] Some fatal errors occurred:
	[ERROR NumCPU]: the number of available CPUs 1 is less than the required 2
[preflight] If you know what you are doing, you can make a check non-fatal with `--ignore-preflight-errors=...`
To see the stack trace of this error execute with --v=5 or higher

* 
X Error starting cluster: run: /bin/bash -c "sudo env PATH=/var/lib/minikube/binaries/v1.18.0:$PATH kubeadm init --config /var/tmp/minikube/kubeadm.yaml  --ignore-preflight-errors=DirAvailable--etc-kubernetes-manifests,DirAvailable--var-lib-minikube,DirAvailable--var-lib-minikube-etcd,FileAvailable--etc-kubernetes-manifests-kube-scheduler.yaml,FileAvailable--etc-kubernetes-manifests-kube-apiserver.yaml,FileAvailable--etc-kubernetes-manifests-kube-controller-manager.yaml,FileAvailable--etc-kubernetes-manifests-etcd.yaml,Port-10250,Swap,SystemVerification": exit status 1
stdout:
[init] Using Kubernetes version: v1.18.0
[preflight] Running pre-flight checks

stderr:
W0502 08:40:46.164113    4607 configset.go:202] WARNING: kubeadm cannot validate component configs for API groups [kubelet.config.k8s.io kubeproxy.config.k8s.io]
	[WARNING IsDockerSystemdCheck]: detected "cgroupfs" as the Docker cgroup driver. The recommended driver is "systemd". Please follow the guide at https://kubernetes.io/docs/setup/cri/
	[WARNING Swap]: running with swap on is not supported. Please disable swap
	[WARNING Service-Kubelet]: kubelet service is not enabled, please run 'systemctl enable kubelet.service'
error execution phase preflight: [preflight] Some fatal errors occurred:
	[ERROR NumCPU]: the number of available CPUs 1 is less than the required 2
[preflight] If you know what you are doing, you can make a check non-fatal with `--ignore-preflight-errors=...`
To see the stack trace of this error execute with --v=5 or higher

* 
* minikube is exiting due to an error. If the above message is not useful, open an issue:
  - https://github.com/kubernetes/minikube/issues/new/choose

When using another driver, it instead looks like:

💡 Requested cpu count 1 is less than the minimum allowed of 2