通过命令yum deplist openldap查看openldap的依赖，可以发现并不需要安装KerxxxDB。
另外，确实是默认带一个数据库，
https://www.openldap.org/doc/admin26/install.html#Database%20Software
OpenLDAP’s slapd(8) MDB primary database backend uses the LMDB software included with the OpenLDAP source. There is no need to download any additional software to have MDB support.