System-owned Instance Metadata

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

Nova doesn't have a way to store arbitrary key/value pairs on an instance that are used internally by the compute service. Such a mechanism would be useful in the development of compute and virt level-plugins that may need to persist state to the DB. In these cases, they could use the instance system metadata table without having to alter the schema using a DB migration.

The proposal is to provide a new table, `instance_system_metdata` that, while almost exactly like `instance_metadata` will not be exposed outside of the compute layer.