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 isn't typo.
`user-data` will be generated automatically in `vagrant up` command.

* Vagrantfile requires `config.rb`: https://github.com/wantedly/nginx-image-server/blob/vagrantfile/Vagrantfile#L25-27
* `user-data` will be generated by `config.rb`: https://github.com/wantedly/nginx-image-server/blob/vagrantfile/config.rb#L4-7