Specify build.jdk and runtime.jdk

Relates elastic/rally#452

cars/v1/vanilla/config.ini

@@ -3,3 +3,7 @@ clean_command=./gradlew clean
 build_command=./gradlew :distribution:archives:tar:assemble
 artifact_path_pattern=distribution/archives/tar/build/distributions/*.tar.gz
 release_url=https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-{{VERSION}}.tar.gz
+# major version of the JDK that is used to build Elasticsearch (only required for plugin builds)
+build.jdk=8
+# list of JDK major versions that are used to run Elasticsearch
+runtime.jdk=8