[EC2] Update default Spark version to 1.2.1

Author: Katsunori Kanda <potix2@gmail.com>

Closes #4566 from potix2/ec2-update-version-1-2-1 and squashes the following commits:

77e7840 [Katsunori Kanda] [EC2] Update default Spark version to 1.2.1

ec2/spark_ec2.py

@@ -42,7 +42,7 @@ from datetime import datetime
 from optparse import OptionParser
 from sys import stderr
 
-SPARK_EC2_VERSION = "1.2.0"
+SPARK_EC2_VERSION = "1.2.1"
 SPARK_EC2_DIR = os.path.dirname(os.path.realpath(__file__))
 
 VALID_SPARK_VERSIONS = set([
@@ -58,6 +58,7 @@ VALID_SPARK_VERSIONS = set([
     "1.1.0",
     "1.1.1",
     "1.2.0",
+    "1.2.1",
 ])
 
 DEFAULT_SPARK_VERSION = SPARK_EC2_VERSION