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.