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

## Current situation

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

```bash
$ 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:

```bash
$ 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.

Start by locating the `set-node` and `use-node` command entry points and review issue #56 for the related command design. Add the `--current-node` behavior shown in the example, preserving the existing node-setting options, and verify that the connectivity command runs against the selected node afterward.

Written by the indexing model from the issue text.