[openstack] Support boot from volume

```
$ bosh-bootstrap deploy
...
1. QCOW2*
2. RAW
Image format to be used: 
```

If `RAW` is selected, then microbosh is configured to use `boot_from_volume`.

NOTE: there are no publicly available RAW stemcells and you must create them yourself https://groups.google.com/a/cloudfoundry.org/d/msg/bosh-users/9GSeE3-wxoc/V95BZVqL45UJ