[EC2] Update mesos/spark-ec2 branch to branch-1.3

[nchammas]

Going forward, we'll use matching branch names across the mesos/spark-ec2 and apache/spark repositories, per the discussion here.

[nchammas]

cc @shivaram

[SparkQA]

Test build #24824 has started for PR 3804 at commit cd2c0d4.

• This patch merges cleanly.

[shivaram]

LGTM

[SparkQA]

Test build #24824 has finished for PR 3804 at commit cd2c0d4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24824/
Test PASSed.

[JoshRosen]

LGTM, too, so I'm going to merge this into master (1.3.0). Thanks!