SPARK-9545, SPARK-9547: Use Maven in PRB if title contains "[test-maven]"

This is just some small glue code to actually make use of the
AMPLAB_JENKINS_BUILD_TOOL switch. As far as I can tell, we actually
don't currently use the Maven support in the tool even though it exists.
This patch switches to Maven when the PR title contains "test-maven".

There are a few small other pieces of cleanup in the patch as well.

Author: Patrick Wendell <patrick@databricks.com>

Closes #7878 from pwendell/maven-tests.