avoid forcing JDK install flavor; use tomcat7 as tomcat user/group

Updating the alfresco recipe to use 4.2.x, I had to change the following things:
1. Use oracle install_flavor (and even installing in my chef logic, I wasn't able to override the assignations in attributes/default.rb)
2. use tomcat7 (and avoid the hardcoded instances of "tomact6" in the recipe); the best solution would be to use alfresco_user and alfresco_group

Thanks a lot for this code, it's awesome!