users: Use own copy of ldapscripts config

Avoid modifying the conffile shipped with ldapscripts.

I tested this change in 2 steps:

1. Running "users setup" action placed the modified file in /etc/ldapscripts/plinth-ldapscripts.conf. However, this also re-ran the slapd setup, which led to ldap being unusable.
2. Starting with a fresh vagrant box, manually placed the file in /etc/ldapscripts/plinth-ldapscripts.conf. I also reverted /etc/ldapscripts/ldapscripts.conf to the original using sudo apt-get --reinstall -o Dpkg::Options::="--force-confask" install ldapscripts. Then I tested user add/modify/delete functions through Plinth.