[COOK-2695] Allow use of oracle java as default version on ubuntu

If you dont wish to use openjdk as your default java version this is currently not possible as apt-get install tomcat6 includes an installation of openjdk-6-jdk.

Since the java cookbook is included before tomcat it will be overridden due to this.

See [ticket](http://tickets.opscode.com/browse/COOK-2695)