Allow to use the bundled JDK for Rally-managed nodes

So far Rally will always download an Elasticsearch version with a bundled JDK (if the distribution contains one) but will explicitly set `JAVA_HOME` when launching a node thus the bundled JDK will never be used for benchmarks. We should investigate how we can allow users to use the bundled JDK (with a command line flag?) and void setting `JAVA_HOME` in any case.