[SPARK-3405] add subnet-id and vpc-id options to spark_ec2.py

Based on this gist:
https://gist.github.com/amar-analytx/0b62543621e1f246c0a2

We use security group ids instead of security group to get around this issue:
https://github.com/boto/boto/issues/350
If this change from `sg` to `sg.id` is due to https://github.com/boto/boto/issues/350, can you add a comment referencing that issue?  Something like 

```
# we use group ids to work around https://github.com/boto/boto/issues/350
```

would be fine.