V3 base boxes

Introduces a simpler approach for building base boxes.

While it does not introduce breaking changes, V3 boxes should be pushing more work into the base box preparation allowing us to eventually have "generic" creation scripts + lxc configs built into vagrant-lxc itself and also to simplify the process of "promoting" existing containers to base vagrant boxes.

V3 base boxes should now be made of 4 files:
 - lxc-template -> The generic creation script
 - lxc.conf -> Default container configs
 - rootfs.tar.gz
 - metadata.json -> Metadata about the box, [required](http://docs.vagrantup.com/v2/boxes/format.html) by Vagrant to work

Because the lxc config will be bundled with the box (and not built during `lxc-create`), I'm not willing to keep editing it from vagrant-lxc, so we'll start setting `lxc.rootfs` and `lxc.utsname` from the provider itself for both V2 and V3 boxes.

**I'm already working on it, just need to find the time to test it and collect feedback from others**