SPARK-1450 [EC2] Specify the default zone in the EC2 script help

This looks like a one-liner, so I took a shot at it. There can be no fixed default availability zone since the names are different per region. But the default behavior can be documented:

```
    if opts.zone == "":
        opts.zone = random.choice(conn.get_all_zones()).name
```

Author: Sean Owen <sowen@cloudera.com>

Closes #3454 from srowen/SPARK-1450 and squashes the following commits:

9193cf3 [Sean Owen] Document that --zone defaults to a single random zone