Packer v1 + LXC clone

[JScott]

I know this repo hasn't changed in a long time but it'd be nice to push these changes upstream if you still want to maintain it. There's a few things that this adds:

• Pulls in Packer v0.9 fix from dliappis' fork
• Changes mitchellh/packer to hashicorp/packer for Packer v1
• Documents deleting vendor dependency steps that I had to do to get it compiling
• Adds "clone_container" parameter

"clone_container" will copy a given LXC container instead of creating a new one and work off of that. It saves us a lot of time in a multi-stage build process for our VM images.

I think the most contentious part is the vendor stuff. I couldn't find another way to get it working though. The data structures used from latest version of multistep and mapstructure aren't compatible with the vendored versions for whatever reason.

[JScott]

We actually did a much better design of this same idea, focused around rootfs tars instead of existing containers. If anyone is considering this then let's chat before you merge I guess.