System-owned server metadata

Mogan already has the concept of server-metadata, however, this is used for *user-provided* metadata.

Currently, Mogan doesn't have a way to store arbitrary key/value pairs for a server which are only used internally by the compute service. For example, Mogan will integrate with RSD to support composing node on the fly, so it is better to save the related field in system metadata which will not be exposed to end-user, while the compute service knows that.

The proposal is to provide a new table, `server_system_metdata` that, while almost exactly like `server_metadata` will not be exposed to end-users.