Given a list of arches, koji can build multiple images in one go
(automatically starting children tasks for each one).
This causes a bunch of changes:
* The configuration no longer allows changing config based on
architecture, only variants are allowed. It is however possible to
filter which arches are used for the building in the variant.
* The configuration files for koji image-build are stored in
work/image-build/$variant (not split based on arch).
This patch also changes the option name that is passed to koji
image-build: the repos should be specified under key `repo` (without the
trailing slash).
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>
