Add Vagrantfile

## WHY

* enable to debug this repo easily

(https://github.com/wantedly/infrastructure/issues/57)

## WHAT

* nginx-image-server on CoreOS VM via Vagrant
* Not using fleet and etcd in CoreOS
* Using "vagrant shared folder"
* Rename `script/cibuild` to `script/test`

@awakia Please try and review this instruction:

```bash
# on your local machine
$ git clone https://github.com/wantedly/nginx-image-server.git && cd nginx-image-server
$ vagrant up
$ vi Dockerfile

# login to VM and test it
$ vagrant ssh
@core-01 $ cd share
@core-01 $ docker build -t target .
@core-01 $ script/test
```
This line comes from upstream repo: 
https://github.com/coreos/coreos-vagrant/blob/master/Vagrantfile#L21

So I prefer to pull request to upstream rather than changing this.
But I think it's enough.