[EC2] Factor out Mesos spark-ec2 branch

[nchammas]

We reference a specific branch in two places. This patch makes it one place.

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[nchammas]

Jenkins, retest this please.

[SparkQA]

Test build #22507 has started for PR 3008 at commit 10a6089.

• This patch merges cleanly.

[SparkQA]

Test build #22507 has finished for PR 3008 at commit 10a6089.

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

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[nchammas]

cc @shivaram@JoshRosen

[shivaram]

LGTM

[nchammas]

cc @JoshRosen@pwendell for final review / merging.

[shivaram]

Actually this is a pretty small change -- I just merged this

[nchammas]

Word. Thanks @shivaram.

[marmbrus]

Just checking, was it intended to leave this out of 1.2?

[shivaram]

Not really. It was just merged after the deadline. I don't think its crucial to have this in 1.2 as its mostly a style change and not a functionality change