[Fun] Introduce StreamSSHCommand function

I thought it would be nice if we could get some output from the various drivers while we're doing provisioning.  E.g. if we call `curl get.docker.com | sh` over SSH it'd be quite nice to see the output of the script as its running to get a feel for how far along it is.  This command will stream the output of a command as it is running.  It is used like:

```go
// suppose we are in digitalocean driver (d)
if err := drivers.streamSSHCommand(d, "echo foo"); err != nil {
    return err
}
if err := drivers.streamSSHCommand(d, "echo bar"); err != nil {
    return err
}
if err := drivers.streamSSHCommand(d, "echo quux"); err != nil {
    return err
}
```

Output:

```
digitalocean | foo
digitalocean | bar
digitalocean | quux
```

Wasn't sure about placement etc. so I put it here.  WDYT?

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>