Allow returning details of snapshots for servers/database servers

You can set an option in either resource's snapshot request that
returns the snapshot's ID as an additional parameter.

This is extended so passing `true` to the model's `#snapshot` methods
returns (via a second API call) the image/database snapshot directly.
As a fog user, I think it makes more sens to return the id of snapshot by default? I can understand if this was done like this (which is just returning `true`) for legacy purposes, but I don't know if any client depends on this behaviour?