updated packer template to install packer onto /dev/sdb1

@YungSang 

Not sure if you care about this or not but if you are we can discuss how to merge.  

This is not a fully working patch because it doesn't include the packer binaries itself.

If you are running on OS X but want to use packer to build docker images (especially during development) you need to have a copy of packer that can run on linux.  The obvious place to put it was inside the boot2docker image so that you can `vagrant ssh -c "/mnt/sdb1/packer blah blah blah"` when you want to build images.  This avoids having two different build schemes one for dev (with `Dockerfile`) and another for deployment with packer when you want to build EC2 images, etc.

I'm pretty sure a better solution would be to have packer understand the `DOCKER_HOST` environment variable but I didn't want to take that project on.

Another small problem is that I don't know enough coreos/systemd mojo to figure out how to get the booted vagrant box to automatically mount `/dev/sdb1` onto `/mnt/sdb1` but you probably know how to do this anyway since you got the original one working correctly for `/dev/sda1`.