[Merge] livecd-rootfs:fix-installer-startup into livecd-rootfs:ubuntu/master

[Sebastien Bacher]

Forwarding a comment from the bug

> Unfortunately a user service cannot really wait for a system service.

which might mean that the fix isn't going to work as intended
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https://code.launchpad.net/~ubuntu-core-dev/livecd-rootfs/+git/livecd-rootfs/+merge/488752
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