Add support for OpenBSD.

Further allow to set the owner/group of the database and slapd.conf file
via parameters. Default values are given in params.pp. This is because
on OpenBSD, ldap owner/group is _openldap:_openldap. Further, change the
permissions of slapd.conf from 0644 to 0640. There can be sensitive
information in that file, i.e. the rootpw, and this should just not
be world readable.
Could we change this to `root` ?