iperf and maybe others need `--enablerepo epel` with AWS Linux

Trying to run iperf with AWS Linux (ami-60b6c60a):

```
./pkb.py --project=anonymous-penguin --benchmarks=iperf --machine_type=t2.nano --cloud=AWS --iperf_sending_thread_count=2 --zone=us-east-1a --image ami-60b6c60a --os_type rhel --aws_user_name ec2-user
```

Took a while, but figured out I needed to set `--aws_user_name ec2-user` and `--os_type rhel` in addition to setting the image. Despite the steps taken in perfkitbenchmarker/linux_packages/iperf.py, `yum install iperf` fails. It works if you add `--enablerepo epel`. I'm not sure, but I'd assume this is harmless to always have enabled.

Related: anyone mind if I create a new wiki page about using AWS Linux, which will list the --aws_user-name and --os_type bits above?