With this commit we derive the appropriate JDK version at benchmark
runtime instead of requiring that it is preconfigured. This requires two
new variables in the rally-teams repo: `build.jdk` and `runtime.jdk` to
resolve the appropriate (major) JDK version depending on the
Elasticsearch version.

When we know the required JDK version, we check the environment
variables `JAVAx_HOME` (where `x` is the JDK version) and `JAVA_HOME` to
find the correct JAVA_HOME path. We also allow the user to override the
runtime JDK with a new command line parameter `--runtime-jdk` as Rally
will default to the highest available and supported JDK version on the
target system.

Closes #452