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
```
> @spesnova looks like you'll need to remove user-data dir from the .gitignore file and put '.gitkeep' in it. Don't forget ignoring everything but .gitignore in the dir.

`user-data` is file, not directory.
There is a possibility that `user-data` includes secret value like `datadog API key`, `AWS secret token` etc... passed from `.env`.
So I think we should not track it same as `.env`.