Move all machine config into `POST isos/` call
Setting machine config in the call to POST machines/ is not useful.
The config is set up correctly on the new machine. But the tests
run against the machine qemu_x86_64 that's predefined on
openqa.gnome.org, and we can't override that. (Setting MACHINE when
starting the job makes the job fail to start).
We should have config in Git as much as possible, so, I have removed
config from the predefined qemu_x86_64 machine and we now set it
in the start_job.sh script that calls POST isos/.