[SPARK-16182][CORE] Utils.scala -- terminateProcess() should call...

[SPARK-16182][CORE] Utils.scala -- terminateProcess() should call Process.destroyForcibly() if and only if Process.destroy() fails

## What changes were proposed in this pull request?

Utils.terminateProcess should `destroy()` first and only fall back to `destroyForcibly()` if it fails. It's kind of bad that we're force-killing executors -- and only in Java 8. See JIRA for an example of the impact: no shutdown

While here: `Utils.waitForProcess` should use the Java 8 method if available instead of a custom implementation.

## How was this patch tested?

Existing tests, which cover the force-kill case, and Amplab tests, which will cover both Java 7 and Java 8 eventually. However I tested locally on Java 8 and the PR builder will try Java 7 here.

Author: Sean Owen <sowen@cloudera.com>

Closes #13973 from srowen/SPARK-16182.