jenkins: use openjdk-7-jre-headless

Set java to version 7 by using Debian 'alternatives' system. That would
make Jenkins start using it.

It is not needed on Jenkins slaves since the Java version to use is
configured in Jenkins.
For jobs, the list of JDK to use is configured in the global Jenkins
configuration and does not depends on Debian alternatives system.

Impact:
- have to restart Jenkins on gallium.wikimedia.org and ensure it works
  fine after.

Change-Id: I7cb798611b2880bf75037397cb3889cde880ad10