Fixes #5782: Use the JDK 1.7+ by default on our packages

Ticket: http://www.rudder-project.org/redmine/issues/5782

To be merged in 3.0
I chose to depend explicitely on OpenJDK since:
- We usually build on pbuilder/pdebuild, which provides no easy facility to install manually an Oracle JDK/JRE
- The "javaX-xdk" provides are inconsistent between versions (java6-sdk, java7-jdk ???)