Allow for an outsider package on RHEL8+

[gcoxmoz]

In openldap::server::config, if redhat 8-or-newer, there's a systemd::dropin_file that makes a startup file so you can tune the user /usr/sbin/slapd runs as.

The problem is, there's a subtle assumption here that your binary is actually /usr/sbin/slapd. Ever since RHEL7.4, the openldap-servers has been deprecated, so some of us have pivoted over to using Symas' packages, which installs everything in /opt. That is, Puppet says to use /usr/sbin/slapd "because you're on RHEL8" (wrong), instead of "because you're using a RHEL-styled package". So this makes it more explicit why you're using this file, and takes it away when you use a different package.

"Why not just symlink slapd?" Tried, didn't work. systemd was not pleased by this. "Why not just make the fully-pathed slapd executable be a parameter which defaults to /usr/sbin/slapd". Thought about it. But for the most part I'm thinking "this file adds unnecessary noise" so I went for the path of least surprise and removed it.