Fix value_for_platform package attributes for ubuntu 14.04.

[jperville]

#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 c4f1570 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).

[bflad]

I'm curious what others think: whether we should default to lxc-docker from the PPA or use the distro package on Ubuntu 14.04? Normally I'd opt for distro default to leave out the "custom" bits, but the lag between releases is pretty far at this point (0.9.1! I only see 0.11.1 in 14.04 testing distro repo too). If I get a community +1 and no -1's, then I'll merge this in or we can discuss further.

[jperville]

I vote for using the ppa by default, since docker is still not production ready (even if 1.0.0 was released recently). To only talk about security, the shocker exploit was only patched on 0.12.0, making ppa package users safe from it but not distro package users.

[djdefi]

For now I will +1 the ppa default, due to the current security landscape and the pace of official distro package updates.

Once the distro's get better at keeping up their official packages, I feel like the distro package is what should be default but it doesn't seem like we are quite there yet.

[bflad]

Good enough for me. Let's roll with it.