Views: apply user SSH public key override
    
    With this patch the SSH public key override attribute is read from the
    FreeIPA server and saved in the cache with the other override data.
    
    Since it is possible to have multiple public SSH keys this override
    value does not replace any other data but will be added to existing
    values.
    
    Fixes https://fedorahosted.org/sssd/ticket/2454
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>