Add remove checkbox-autostart file feature for vivid systemd file

As mentioned in this merge request[1]:
Current mini CI system will turn itself off after finishing the test, but it will become a problem for SRU system, which will be controlled by SSH.

What we need is to purge the checkbox-autostart-desktop and checkbox-autostart-server package, it will stop checkbox sru to start on boot.

However, this purge prove to be not working, as it will also remove the ci-mailer script that we use for submitting results.

We will need to remove the autostart file instead.

[1] https://code.launchpad.net/~taihsiangho/checkbox/checkbox-150611-rm-auto-after-minici/+merge/261807