[SPARK-16257][BUILD] Update spark_ec2.py to support Spark 1.6.2 and 1.6.3.

## What changes were proposed in this pull request?

- Adds 1.6.2 and 1.6.3 as supported Spark versions within the bundled spark-ec2 script.
- Makes the default Spark version 1.6.3 to keep in sync with the upcoming release.
- Does not touch the newer spark-ec2 scripts in the separate amplabs repository.

## How was this patch tested?

- Manual script execution:

export AWS_SECRET_ACCESS_KEY=_snip_
export AWS_ACCESS_KEY_ID=_snip_
$SPARK_HOME/ec2/spark-ec2 \
    --key-pair=_snip_ \
    --identity-file=_snip_ \
    --region=us-east-1 \
    --vpc-id=_snip_ \
    --slaves=1 \
    --instance-type=t1.micro \
    --spark-version=1.6.2 \
    --hadoop-major-version=yarn \
    launch test-cluster

- Result: Successful creation of a 1.6.2-based Spark cluster.

This contribution is my original work and I license the work to the project under the project's open source license.

Author: Brian Uri <[email protected]>

Closes apache#13947 from briuri/branch-1.6-bug-spark-16257.

(cherry picked from commit ccc7fa3)

Author: Brian Uri

ec2/spark_ec2.py

@@ -51,7 +51,7 @@
     raw_input = input
     xrange = range
 
-SPARK_EC2_VERSION = "1.6.1"
+SPARK_EC2_VERSION = "1.6.3"
 SPARK_EC2_DIR = os.path.dirname(os.path.realpath(__file__))
 
 VALID_SPARK_VERSIONS = set([
@@ -77,6 +77,8 @@
     "1.5.2",
     "1.6.0",
     "1.6.1",
+    "1.6.2",
+    "1.6.3",
 ])
 
 SPARK_TACHYON_MAP = {
@@ -96,14 +98,16 @@
     "1.5.2": "0.7.1",
     "1.6.0": "0.8.2",
     "1.6.1": "0.8.2",
+    "1.6.2": "0.8.2",
+    "1.6.3": "0.8.2",
 }
 
 DEFAULT_SPARK_VERSION = SPARK_EC2_VERSION
 DEFAULT_SPARK_GITHUB_REPO = "https://github.com/apache/spark"
 
 # Default location to get the spark-ec2 scripts (and ami-list) from
 DEFAULT_SPARK_EC2_GITHUB_REPO = "https://github.com/amplab/spark-ec2"
-DEFAULT_SPARK_EC2_BRANCH = "branch-1.5"
+DEFAULT_SPARK_EC2_BRANCH = "branch-1.6"
 
 
 def setup_external_libs(libs):