title: Install multiple .service files with dh_systemd packaging
body: <p>I'm currently packaging a python app with dh_virtualenv, daemonized with systemd.</p>
  <p>I use the dh_systemd plugin to automatically install the file my_app.service will install the .deb package, but I'd like to run another process in a different service that schedules tasks for my app, using celery.</p>
  <p>So, I created another service file, my_app.scheduler.service, but I don't know how to declare this file/app/service in my debian packaging rules, so that while installing the whole package, both services file will be copied and thus will be launched independently.</p>
  <p>Here are my debian conf files for dpkg-buildpackage command :</p>
  <p>debian/control</p>
  <pre><code>Source: my_app
  Section: python
  Priority: extra
  Maintainer: me
  Build-Depends: debhelper (&gt;= 9), python, dh-virtualenv (&gt;= 0.6), dh-systemd (&gt;= 1.5), adduser
  Standards-Version: 3.9.5
  
  Package: my_app
  Architecture: any
  Pre-Depends: dpkg (&gt;= 1.16.1), python2.7, ${misc:Pre-Depends}
  Depends: ${python:Depends}, ${misc:Depends}
  Description: blabla
  </code></pre>
  <p>debian/rules:</p>
  <pre><code>#!/usr/bin/make -f
  
  %:
      dh $@ --with python-virtualenv --with=systemd
  </code></pre>
  <p>debian/install</p>
  <pre><code>etc/my_app.config /etc/
  </code></pre>
  <p>debian/dirs:</p>
  <pre><code>/var/lib/my_app
  /var/log/my_app
  </code></pre>
  <p>And of course the .service files:</p>
  <p>debian/my_app.service</p>
  <pre><code>[Unit]
  Description=APP
  
  [Service]
  Type=simple
  User=app_user
  EnvironmentFile=/etc/my_app.config
  ExecStart=/usr/share/python/my_app/bin/gunicorn -w 10 -b 0.0.0.0:6000 -t 600 my_app_python_package:app
  
  [Install]
  WantedBy=multi-user.target
  </code></pre>
  <p>debian/my_app.scheduler.service</p>
  <pre><code>[Unit]
  Description=APP Scheduler
  
  [Service]
  Type=simple
  User=app_user
  EnvironmentFile=/etc/my_app.config
  ExecStart=/usr/share/python/my_app/bin/celery worker -A my_app_python_package.tasks 
  
  [Install]
  WantedBy=multi-user.targetroot
  </code></pre>