Upgrade docker to 1.1.2 and add docker module

This version of module has disabled socketActivation, because until
nixos upgrade systemd to at least 214, systemd does not support
SocketGroup. So socket is created with "root" group when
socketActivation enabled. Should be fixed as soon as systemd upgraded.

Includes changes from #3015 and supersedes #3028

(It's my first nixos module, so feel free to critique me)