Add `docker build` Dockerfile for ldap systemd container

Without this patch there is no single Dockerfile that implements a build
process that results in a working LDAP container.  This patch addresses
the problem by using the centos7vps base image, which includes systemd,
to build a LDAP service container.

The LDAP service can be started, mapping 389 on the host to 389 in the
container:

    CID=$(docker run -p 127.0.0.1:389:389 --privileged -d ldap_sysd)

This patch modifies the original Puppet code for the shared jenkins+ldap
VM overriding the Service[slapd] resource.  This is necessary because
Puppet defaults to systemd service management while systemd is not
running during the build phase.  The override manually starts the LDAP
process as a dependency of loading the initial schema and branches.

This approach differs from the approach of "side-loading" [1] the data
using two containers at runtime.

[1]: http://jpetazzo.github.io/2014/06/23/docker-ssh-considered-evil/