ipa-backup: Add mechanism to store empty directory structure
    
    Certain subcomponents of IPA, such as Dogtag, cannot function if
    non-critical directories (such as log directories) have not been
    stored in the backup.
    
    This patch implements storage of selected empty directories,
    while preserving attributes and SELinux context.
    
    https://fedorahosted.org/freeipa/ticket/5297
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>