[SPARK-2912] [Spark QA] [WIP] Include commit hash in Spark QA messages

You can find the [discussion that motivated this PR here](http://mail-archives.apache.org/mod_mbox/spark-dev/201408.mbox/%3CCABPQxssy0ri2QAz=cc9Tx+EXYWARm7pNcVm8apqCwc-esLbO4Q@mail.gmail.com%3E). Note that this is a work in progress and will be replaced by a cleaner PR.

As described in [SPARK-2912](https://issues.apache.org/jira/browse/SPARK-2912), the goal of this PR (and related ones to come) is to include useful detail in Spark QA's messages that are intended to make a committer's job easier to do.

Since this work depends on Jenkins, I cannot test this locally. Hence, I will be iterating via this PR.

[Note: This is a duplicate of a [previous PR](https://github.com/apache/spark/pull/1811), without the extraneous commits.]
The "tests finished" message includes whether the patch was merge-able or not. Would you like to track that in the "tests started" message as well? It should be the same across the start and finish messages for a given test cycle, so I think it's redundant.