Add support for <cinder type-list> and <cinder create --volume-type>

Added support for volume types
It doesn't expect them within a encoded string without being escaped. They are, however, acceptable outside. Example:

```python
import json

s = """{
    "firstName": "John",
    "lastName": "Smith",
    "isAlive": true,
    "age": 25,
    "height_cm": 167.6,
    "address": {
        "streetAddress": "21 2nd Street",
        "city": "New York",
        "state": "NY",
        "postalCode": "10021-3100"
    },
    "phoneNumbers": [
        { "type": "home", "number": "212 555-1234" },
        { "type": "office",  "number": "646 555-4567" }
    ]
}"""

print json.loads(s)
```