[SPARK-874] adding a --wait flag

[jbencook]

This PR adds a --wait flag to the ./sbin/stop-all.sh script.

[AmplabJenkins]

Can one of the admins verify this patch?

[pwendell]

Jenkins, test this please. LGTM pending tests.

[SparkQA]

Test build #24252 has started for PR 3567 at commit d05c5bb.

• This patch merges cleanly.

[SparkQA]

Test build #24252 has finished for PR 3567 at commit d05c5bb.

• 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/24252/
Test PASSed.

[pwendell]

Thanks Ben - I merged this.

[jbencook]

Awesome!