SPARK-1714. Take advantage of AMRMClient APIs to simplify logic in YarnA...

...llocator

The goal of this PR is to simplify YarnAllocator as much as possible and get it up to the level of code quality we see in the rest of Spark.

In service of this, it does a few things:
* Uses AMRMClient APIs for matching containers to requests.
* Adds calls to AMRMClient.removeContainerRequest so that, when we use a container, we don't end up requesting it again.
* Removes YarnAllocator's host->rack cache. YARN's RackResolver already does this caching, so this is redundant.
* Adds tests for basic YarnAllocator functionality.
* Breaks up the allocateResources method, which was previously nearly 300 lines.
* A little bit of stylistic cleanup.
* Fixes a bug that causes three times the requests to be filed when preferred host locations are given.

The patch is lossy. In particular, it loses the logic for trying to avoid containers bunching up on nodes. As I understand it, the logic that's gone is:

* If, in a single response from the RM, we receive a set of containers on a node, and prefer some number of containers on that node greater than 0 but less than the number we received, give back the delta between what we preferred and what we received.

This seems like a weird way to avoid bunching E.g. it does nothing to avoid bunching when we don't request containers on particular nodes.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3765 from sryza/sandy-spark-1714 and squashes the following commits:

32a5942 [Sandy Ryza] Muffle RackResolver logs
74f56dd [Sandy Ryza] Fix a couple comments and simplify requestTotalExecutors
60ea4bd [Sandy Ryza] Fix scalastyle
ca35b53 [Sandy Ryza] Simplify further
e9cf8a6 [Sandy Ryza] Fix YarnClusterSuite
257acf3 [Sandy Ryza] Remove locality stuff and more cleanup
59a3c5e [Sandy Ryza] Take out rack stuff
5f72fd5 [Sandy Ryza] Further documentation and cleanup
89edd68 [Sandy Ryza] SPARK-1714. Take advantage of AMRMClient APIs to simplify logic in YarnAllocator