Should be able to specify latest version of a branch for tests

[xkrogen]

The Hadoop tarball to use during testing is specified and downloaded from an Apache mirror. Generally only the latest version of each branch is available, but right now, versions must be fully specified (e.g. 2.7.4), so they will go out of date as new maintenance releases come into existence. It should be possible to specify like e.g. 2.7.*