Fixed distcheck failure due to test_containerizer.py missing in
EXTRA_DIST.

Review: https://reviews.apache.org/r/20784
Added count to metric statistics.

Review: https://reviews.apache.org/r/20733
[MESOS-1252]: Support ENV MAVEN_HOME on the build.

The build now supports the `MAVEN_HOME` environment variable which is
used to resolve the `mvn` executable. If no `MAVEN_HOME` is specified
we will use the one available in the `PATH`.

Review: https://reviews.apache.org/r/20750