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.

ec2/spark_ec2.py

@@ -234,10 +234,10 @@ def get_spark_ami(opts):
         "cg1.4xlarge": "hvm",
         "hs1.8xlarge": "pvm",
         "hi1.4xlarge": "pvm",
-        "m3.medium":   "pvm",
-        "m3.large":    "pvm",
-        "m3.xlarge":   "pvm",
-        "m3.2xlarge":  "pvm",
+        "m3.medium":   "hvm",
+        "m3.large":    "hvm",
+        "m3.xlarge":   "hvm",
+        "m3.2xlarge":  "hvm",
         "cr1.8xlarge": "hvm",
         "i2.xlarge":   "hvm",
         "i2.2xlarge":  "hvm",