Enhance edge gateway creation with more GatewayBackingConfig values

Edge gateway creation (#130) uses only two values for GatewayBackingConfig: "full" and "compact". The GUI mentions other values "Large", "Xlarge", "Quad Large" which are not in the API.

When we try to enter values other than "full" or "compact" in the creation, we get an error from vCD saying that such values are not supported.

We need to figure out in which version these values are supported and what is the minimum version for our API to handle such requests.

Our current assessment is that the rejection happens because our current API minimal version (27) is insufficient for handling the request.

We will revisit this issue when we bump up go-vcloud-director minimal version.