export: fix systemd dependencies

With the previous way systemd services were exported, you had to enable every single process one by one to get the service operational.

Consider the following Procfile:

    web: bundle exec thin start
    worker: bundle exec worker start

Exported with:

    foreman export systemd /usr/lib/systemd/system -a myapp -c worker=3

You will have to perform the following to start the service:

    systemctl enable myapp-web-1.service
    systemctl enable myapp-worker-1.service
    systemctl enable myapp-worker-2.service
    systemctl enable myapp-worker-3.service
    systemctl enable myapp-web.target
    systemctl enable myapp-worker.target
    systemctl start myapp.target

Thats 7 commands, and you have to know the names of each service. Nasty.

With the changes here, you will have to perform the following:

    systemctl enable myapp.target
    systemctl start myapp.target

You can also `systemctl start myapp.target` without enabling as well. Previously if you tried to start `myapp.target` without enabling each individual process it would not work.

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Additionally, this change also adds the dependency to 'multi-user.target' (the default behavior for all services). This will properly shut down (or start) the app when the system changes runlevels. The previous method did not do this.
Merge commit 'bc282ca068a8c82516250783679aac3ed6cee7a5' into cloudcom

* commit 'bc282ca068a8c82516250783679aac3ed6cee7a5':
  export: fix systemd dependencies