def determine(distro, release)
      return distro_releases[[distro, release]] if distro_releases.has_key? [distro, release]
      return distros[distro] if distros.has_key? distro
      default
    end
def determine(distro, release)
      return distro_releases[[distro, release]] if distro_releases.has_key? [distro, release]
      return distros[distro] if distros.has_key? distro
      default
    end
Evaluation which packages (with version constraint) are needed for the given distrobution
 release.
 @param distro[Symbol] The distribution name like :ubuntu, :debian ...
 @param release[Symbol] The distribution release like :precise for ubuntu 12.04
 @return [Array<String, String>]