Ticket #47693 - Environment variables are not passed when DS is started via service

Description: Environment variables (except TERM and LANG) are ignored
if a program is started via service.

If it is started with systemctl, it takes this COMMAND and the values
are correctly passed to the server.
systemctl set-environment SLAPD_MXFAST=0 MALLOC_TRIM_THRESHOLD_=4096

To control them explicitly and to provide the same instructions to
the service and systemctl, it'd be good to have some variables
(SLAPD_MXFAST, MALLOC_TRIM_THRESHOLD_ and MALLOC_MMAP_THRESHOLD_ in
this patch) configurable.

https://fedorahosted.org/389/ticket/47693

Reviewed by rmeggins@redhat.com (Thank you, Rich!!)
(cherry picked from commit 02169effb14ea082cdfed5377244b4ffedb7c437)