Make the migration plugin more configurable
    
    This patch adds new options to the migration plugin:
     * the option to fine-tune the objectclass of users or groups being imported
     * the option to select the LDAP schema (RFC2307 or RFC2307bis)
    
    Also makes the logic that decides whether an entry is a nested group or user
    (for RFC2307bis) smarter by looking at the DNS. Does not hardcode primary keys
    for migrated entries.
    
    https://fedorahosted.org/freeipa/ticket/429