LXC 3.0, unlike 2.1, doesn't support old configuration formats at all
and because our previous approach to handling old format was incomplete,
running `vagrant up` on a system with LXC 3.0 would fail, even if the
base box itself was made with modernized config.
This commit fixes this problem: is the box has a modern config, it's
going to be properly created and booted under LXC 3.0. It does so
without breaking the support for LXC < 2.1