3. core: Simple parameters for add and remove users

The commands that add/remove users and groups receive as parameters
the complete entities that have to be added. This patch changes them to
use simple parameters.

Change-Id: I5c37a686b28f38ce47e9a7d590e3f706164dc09c
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>