set attributs compabible for Chef 11.0
server: create preseeding directory
suggests openssl + nscd, only needed for openldap::auth
use attributes instead of hard-coded value for global slapd options

Replaces global slapd.conf hard-coded values from slapd.conf-template
with node attributes.
A bigger refactoring to support full configuration via node attributes
(so that not cookbook editing is needed to adjust slapd for your needs)
is planed and will follow with additional commits.