slapd fails to install if /etc/ldap/slapd.conf is already there

I am trying to install `slapd` on precise, on a machine that was
freshly installed (thus, no slapd previously installed).

I copy `slapd.conf` over from the old server.

Then I try to install `slapd` in a non-interactive fashion (done
through CFEngine 3), using this command:

        /usr/bin/env DEBIAN_FRONTEND=noninteractive LC_ALL=C /usr/bin/aptitude -o Dpkg::Options::=--force-confold -o Dpkg::Options::=--force-confdef --assume-yes install

However, this fails with the following errors:

    Setting up slapd (2.4.28-1.1ubuntu4) ...
      Creating initial configuration... mkdir: cannot create directory `/etc/ldap/slapd.conf': File exists

So apparently the postinst script guesses the correct value for
SLAPD_CONF but: (1) still treats it as if it were a directory (i.e.,
`SLAPD_CONF=sladp.d`), and (2) tries to create it as if it did not
exist.