Switch pipeline to 'architectures'

'arch_override' naming is less clear so let's switch it over. Trigger configs now have both variables so we can make the switch.

While're at it, disallow 'architectures' not being specified. We really need this variable to be explicit for each pipeline (which si the case already), as there's no setup where all architectures in the list are supported.

Signed-off-by: Veronika Kabatova vkabatov@redhat.com