docker 1.10 doesn't properly use systemd docker integration

[philips]

Docker 1.9 used systemd integration by default for cgroups. Docker 1.10 changes this default via this libcontainer PR: moby/moby#17704

We need to add this new exec-opt flag to our docker daemon systemd unit:

docker daemon --exec-opt native.cgroupdriver=systemd

cc @aaronlevy @mischief

[marineam]

Should be fixed in this week's alpha.