Make sure that RancherOS is fully compatible with Rancher 2.0

[niusmallnan]

As we know that Rancher 2.0 will be released soon, we should ensure that RancherOS can run it well.

If possible, we should provide a guide to point out some key configurations.

[niusmallnan]

From hgalal:
actually the prefix path should work out of the box with rancherOS, rke automatically detect RancherOS and change the prefix to /opt/rke, which means the certificates for example will be saved in /opt/rke/etc/kubernetes/ssl

RKE can work with RancherOS now.

[niusmallnan]

From @kingsd041

1. provisioning to AWS using RancherOS 1.4.0-rc2 in Rancher 2.0

case1: Test canal/flannel/calico network pass
case2: Test kubernetes v1.10.1-rancher2-1/v1.9.5-rancher1-1/v1.8.10-rancher1-1 pass
case3: Test longhorn fail
case4: Test nfs-provisioner fail

2. provisioning to VSphere using RancherOS-VMware 1.4.0-rc2 in Rancher 2.0

case1: Test canal/flannel/calico network pass
case2: Test kubernetes v1.10.1-rancher2-1/v1.9.5-rancher1-1/v1.8.10-rancher1-1 pass
case3: Test longhorn fail
case4: Test nfs-provisioner fail

[janeczku]

We shall document some key configurations for using RancherOS with Rancher 2.0 (including the VSphere node driver). @niusmallnan