Set JAVA12_HOME

With this commit we set the environment variable `JAVA12_HOME` so Rally
can pick it up in order to compile and run with JDK 12.

Relates elastic#97

Author: danielmitterdorfer

night_rally/fixtures/ansible/roles/rally-update/tasks/main.yml

@@ -30,5 +30,6 @@
     JAVA9_HOME: /var/lib/jenkins/.java/java9
     JAVA10_HOME: /var/lib/jenkins/.java/java10
     JAVA11_HOME: /var/lib/jenkins/.java/java11
+    JAVA12_HOME: /var/lib/jenkins/.java/openjdk12
   become: true
   become_user: jenkins