Fix value_for_platform package attributes for ubuntu 14.04.

#168 allowed to override the name of the docker package to install, with default of `lxc-docker` for all ubuntu and debian platform. However, the refactoring done in c4f1570356471c6c220b2c0b7615bd73106f1f0f broke it for me, because I run ubuntu 14.04 which is not listed in the `value_for_platform` cases.

This PR fixes it for ubuntu 14.04, but does not handle the upcoming 14.10 and later releases (which are likely to be affected too).