Add flag `--current-node` to `set-node` command

[blanquicet]

Current situation

Users have to set the node and set it as current node:

$ kubectl aks config set-node node1 --subscription mySubID --node-resource-group myRG --vmss myVMSS --instance-id myInstanceID1
$ kubectl aks config use-node node1

# Run command automatically on node1
$ kubectl aks check-apiserver-connectivity

Ideal situation

Allow doing it in one command:

$ kubectl aks config set-node node1 --current-node --subscription mySubID --node-resource-group myRG --vmss myVMSS --instance-id myInstanceID1

# Run command automatically on node1
$ kubectl aks check-apiserver-connectivity

NOTE

Consider changing use-node to set --current-node. Specially after https://github.com/Azure/kubectl-aks/issues/56.