Creating a new vagrant machine with a box created with an old LXC config
format would fail with LXC 3.0. We now update the LXC config of the box
itself before calling `lxc-create`.
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
When running with LXC 2.1+, ensure that our boxes have up-to-date config
keys by running the `lxc-update-config` utility that ships with LXC
2.1+.
When the command doesn't exist (LXC <2.1), we do nothing.
ref #445
