The test schema for volumes had `storage_type` as a `String` which was
not being returned by the mock implementation.
According to the API docs I checked, it is an allowed value (but the
returned fields are controlled by the request).
So this makes the type `Fog::Nullable::String` so the schema passes if
the key/value are absent.
There appears to be a number of bugs in the CloudSigma mock interface.
The `snapshots` key is `nil` and adding a new snapshot fails. Working
around that, the tests timeout because there is no mechanism to change
the snapshots state from "creating" so it never becomes `available?`
