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.
Revert "[maven-release-plugin] prepare for next development iteration"

This reverts commit 865e6f63f63f5e881a02d1a4e3b4c5d0e86fcd8e.
Revert "[maven-release-plugin] prepare release v1.1.0-rc3"

This reverts commit b2d0493b223c5f98a593bb6d7372706cc02bebad.