[SPARK-2065] give launched instances names

This update resolves [SPARK-2065](https://issues.apache.org/jira/browse/SPARK-2065). It gives launched EC2 instances descriptive names by using instance tags. Launched instances now show up in the EC2 console with these names.

I used `format()` with named parameters, which I believe is the recommended practice for string formatting in Python, but which doesn’t seem to be used elsewhere in the script.
Do you mind wrapping this before 100 char wide? We don't yet enforce it, but we will soon enforce line length in Python as well. Also for line 425. Thanks!