User life cycle: stageuser-add verb
    
    Add a accounts plugin (accounts class) that defines
    variables and methods common to 'users' and 'stageuser'.
    accounts is a superclass of users/stageuser
    
    Add the stageuser plugin, with support of stageuser-add verb.
    
    Reviewed By: David Kupka, Martin Basti, Jan Cholasta
    
    https://fedorahosted.org/freeipa/ticket/3813
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: David Kupka <dkupka@redhat.com>