CLI `switch` command is backwards

Instead of

```
mininet> switch s1 start
```

we should allow

```
mininet> switch start s1 s2 s3 [...]
```

alternately we _could_ support

```
mininet> switch s1 s2 s3 start
```

but... is this really better? It seems having the verb/command at the front makes it easier to see what is going on.
