[SPARK-3391][EC2] Support attaching up to 8 EBS volumes.

Please merge this at the same time as https://github.com/mesos/spark-ec2/pull/66

Author: Reynold Xin <rxin@apache.org>

Closes #2260 from rxin/ec2-ebs-vol and squashes the following commits:

b9527d9 [Reynold Xin] Removed io1 ebs type.
bf9c403 [Reynold Xin] Made EBS volume type configurable.
c8e25ea [Reynold Xin] Support up to 8 EBS volumes.
adf4f2e [Reynold Xin] Revert git repo change.
020c542 [Reynold Xin] [SPARK-3391] Support attaching more than 1 EBS volumes.