It makes no sense to repeat the same configuration for multiple
architectures. Instead we should just list the architectures as another
key in the mapping. There is an option to specify multiple config dicts.
This preserves full backwards compatibility, the old config format is
still accepted.
Fixes: https://pagure.io/pungi/issue/678
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>