Allow user overrides to specify SSH public keys
    
    Overrides for users can have SSH public keys. This, however, will not enable
    SSH public keys from overrides to be actually used until SSSD gets fixed to
    pull them in.
    
    SSSD ticket for SSH public keys in overrides:
    https://fedorahosted.org/sssd/ticket/2454
    
    Resolves https://fedorahosted.org/freeipa/ticket/4509
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>