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

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.

Author: Andrew Or <andrew@databricks.com>

Closes #3615 from andrewor14/dynamic-allocation-yarn-only and squashes the following commits:

ce6487a [Andrew Or] Allow requesting / killing executors only in YARN mode