Fix systemd startup service issue

[Ph-ill]

When starting tinyticker as a systemd service without specifying the user the service fails to start as the tinyticker pip module is not installed for the root user. ie.

ModuleNotFoundError: No module named 'tinyticker'

[loiccoyle]

The service files in this repo are not the same service files found on the disk image.

The disk image files are found here:
https://github.com/loiccoyle/pi-gen/tree/master/stage2/05-tinyticker/files

I would like to keep the ones in this repo to be "universal" and not rely on the tinyticker user existing as it does in the disk image. They are here to help ppl who don't use the provided disk image basically and want to setup tinyticker on their own.

Also the service files in the disk image are installed as services of the tinyticker user and as such I don't think adding the user line does anything.

I will close this PR, feel free to open one on the linked pi-gen repo if you want to make changes to the service files in the disk image.