Hardcoded temp file prevents simultaneous runs by different users

[Max Brustkern]

When I'm running the tests, this file is generated:
/tmp/ubuntu-system-tests-temp.conf
If I try to run tests as more than one user at a time, the second user will fail because it cannot create that file. I assume if I try to run multiple tests at the same time as the same user (which I do every day as part of automation,) any differences in configuration across the different tests would be lost.