[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
Minor grammar / style nit, but what do you think about "VPC subnet to launch instances in" as the help text?