provisioners/ansible: use Docker proxy when needed

Close #4071

@jabclab could you please tell us if you have time to review this on monday. Yeah, sorry for the very short delay, but @mitchell aims at shipping Vagrant 1.7 next week!

PS: unit tests are missing for the moment, due to personal lack of time. If it's ok for you, I'll implement them after #4883 being integrated and when resuming my work on #2103.