Add docker.io for Ubuntu 14.04.1 LTS (trusty) 

Hi,

May I suggest some improvments?
Mostly it should not change anything for Ubuntu version prior to Ubuntu 14.04.1 LTS (trusty).
I have added some parameters in order to be easily overrided.
But for Ubuntu 14.04.1 LTS (trusty) I have added docker.io that I found on the following ansible script
https://github.com/jamesdbloom/ansible-install-docker.

Thanks,

Alban