SPARK-4843 [YARN] Squash ExecutorRunnableUtil and ExecutorRunnable

ExecutorRunnableUtil is a parent of ExecutorRunnable because of the yarn-alpha and yarn-stable split. Now that yarn-alpha is gone, this commit squashes the unnecessary hierarchy. The methods from ExecutorRunnableUtil are added as private.
In the old code, `prepareEnvironment` was called after `yarnConf` had been assigned:

```scala
trait ExecutorRunnableUtil extends Logging {
 val yarnConf: YarnConfiguration
 val sparkConf: SparkConf
 lazy val env = prepareEnvironment
```

Here, though, `env` appears before `yarnConf`.  I don't think that this is a problem here since `env` is a `lazy val`, but I wonder if we should move it as late in the `vals` section as possible so that its dependencies are declared before it can run.