restapi: Add external id to users and groups

Eairlier changes have added the external identifier field to the user
and group entities, but it wasn't returned by the RESTAPI yet. This
patch adds support for that.

Change-Id: I799219080a57c6ec4ef0e16c742f1f44d11f1080
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>