Use new classes for run_command and Service
    
    User and Group now return unmodified instance when they are called with
    an instance of themselves: User(user) is user.
    
    run_command() and Service class accept either names or User object.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>