[SPARK-4215] Allow requesting / killing executors only in YARN mode

[andrewor14]

Currently this doesn't do anything in other modes, so we might as well just disable it rather than having the user mistakenly rely on it.

[SparkQA]

Test build #24170 has started for PR 3615 at commit ce6487a.

• This patch merges cleanly.

[sryza]

+1

[SparkQA]

Test build #24170 has finished for PR 3615 at commit ce6487a.

• 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/24170/
Test PASSed.

[andrewor14]

Ok, I'll merge this into master. thanks