Fix assignment KVS backend to not use identity

The assignment KVS backend was using the identity API. This change
makes it so that the KVS backend does not use the identity API.

The assignment KVS backend was using the identity API to:

a) Store the user's projects.

The user's projects were also kept in the metadata. So the KVS
backend was changed to use the metadata rather than the user data.

a) Determine if the ID in the metadata was for a user or group.

The metadata for users and groups was split into separate keys.
Metadata for users is now prefixed with `metadata_user-` and
`metadata-groups-` is used for groups.

bp no-check-id

Change-Id: I5cc01d203d12b15575b72ad57aa3820d1d36d09f
Merge "Fix assignment KVS backend to not use identity"