add `bucket` option in `gantree init`

resolve #20, now is possible send bucket name to `gantree init`:

```bash
$ gantree init gullitmiranda/docker-deploy:ruby -b aircrm-docker
```

Options:
```bash
$ gantree help init

Usage:
  gantree init IMAGE

Options:
  -u, [--user=USER]      # user credentials for private repo
  -p, [--port=PORT]      # port of running application
  -b, [--bucket=BUCKET]  # set bucket name, default is 'docker-cfgs'

```