SPARK-1450 [EC2] Specify the default zone in the EC2 script help

[srowen]

This looks like a one-liner, so I took a shot at it. There can be no fixed default availability zone since the names are different per region. But the default behavior can be documented:

    if opts.zone == "":
        opts.zone = random.choice(conn.get_all_zones()).name

[SparkQA]

Test build #23837 has started for PR 3454 at commit 9193cf3.

• This patch merges cleanly.

[SparkQA]

Test build #23837 has finished for PR 3454 at commit 9193cf3.

• 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/23837/
Test PASSed.