apache directory server: do not start with systemctl

Environment:
--------------

on ubuntu 16.04.2 TLS

Issue:
--------
systemctl start apacheds.service
Failed to start apacheds.service: Unit apacheds.service is not loaded properly: Invalid argument.
See system logs and 'systemctl status apacheds.service' for details.

Steps to reproduce:
------------------

- install oracle jdk8 :
apt install oracle-java8-installer oracle-java8-unlimited-jce-policy oracle-java8-set-default
- install apacheds:
apt install apacheds
- start apacheds:
  systemctl enable apacheds.service
  systemctl start apacheds.service

Logs/output/files:
------------
root@security:~# systemctl status apacheds.service
● apacheds.service - Apache Directory Server
   Loaded: error (Reason: Invalid argument)
   Active: inactive (dead)

Mar 25 11:14:04 security systemd[1]: [/lib/systemd/system/apacheds.service:11] Executable path is not absolute, ignoring: ${JAVA_HOME}/bin/java ${JAVA_OPTS} -Dapacheds.controls=${ADS_CONTROLS}
Mar 25 11:14:04 security systemd[1]: apacheds.service: Service lacks both ExecStart= and ExecStop= setting. Refusing.

Root cause
----------

 - first: apacheds.service contains syntax error
systemctl edit apacheds.service:

with won't work:
ExecStart=${JAVA_HOME}/bin/java ${JAVA_OPTS} \
            -Dapacheds.controls=${ADS_CONTROLS} \
            -Dapacheds.extendedOperations=${ADS_EXTENDED_OPERATIONS} \
            -Dlog4j.configuration=file:${ADS_INSTANCES}/${ADS_INSTANCE}/conf/log4j.properties \
            -Dapacheds.log.dir=${ADS_INSTANCES}/${ADS_INSTANCE}/log \
            -cp '${ADS_HOME}/lib/*' \
            org.apache.directory.server.UberjarMain \
            ${ADS_INSTANCES}/${ADS_INSTANCE}/

this works fine:
ExecStart=/bin/sh -c "${JAVA_HOME}/bin/java ${JAVA_OPTS} \
            -Dapacheds.controls=${ADS_CONTROLS} \
            -Dapacheds.extendedOperations=${ADS_EXTENDED_OPERATIONS} \
            -Dlog4j.configuration=file:${ADS_INSTANCES}/${ADS_INSTANCE}/conf/log4j.properties \
            -Dapacheds.log.dir=${ADS_INSTANCES}/${ADS_INSTANCE}/log \
            -cp '${ADS_HOME}/lib/*' \
            org.apache.directory.server.UberjarMain \
            ${ADS_INSTANCES}/${ADS_INSTANCE}/"

 - second:
   /usr/lib/jvm/default-java do not exists if oracle jdk is installed: i had to create it:
   cd /usr/lib/jvm/ && ln -s java-8-oracle default-java