SPARK-3358: [EC2] Switch back to HVM instances for m3.X.

During regression tests of Spark 1.1 we discovered perf issues with
PVM instances when running PySpark. This reverts a change added in #1156
which changed the default type for m3 instances to PVM.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #2244 from pwendell/ec2-hvm and squashes the following commits:

1342d7e [Patrick Wendell] SPARK-3358: [EC2] Switch back to HVM instances for m3.X.