Add openstack image fields

Add openstack specific fields (minDisk, minRam, metadata).

The first two fields ("minDisk", "minRam") are similar to [Joyent's "requirements"](https://github.com/pkgcloud/pkgcloud/blob/master/lib/pkgcloud/joyent/compute/image.js#L32). If other clouds also provide them, maybe pkgcloud could expose these properties under the same name (e.g. "requirements": { "minRam": 10, "minDisk": 20 })