Ubuntu 14.04

by this I mean, if you edit he linked file to add a os_ver override, then in rubber*.yml you could have:
roles:
   packages: [foo]
os_ver:
  14.04:
     packages: [newfoo]


maybe expand the syntax for variable merging similar to:
https://github.com/rubber/rubber/blob/master/lib/rubber/environment.rb#L122
Which forces the override to fully replace instead of merge