Use architecture from LABEL not the one which buildroot

[pbabinca]

Architecture LABEL isn't passed to children tasks and architecture of the buildroot is used instead.

[ausil]

The LABEL handling likely needs to be moved to the child task entirely

take https://github.com/release-engineering/koji-containerbuild/blob/master/koji_containerbuild/plugins/builder_containerbuild.py#L785

take output_template that gets sent to the child tasks. it has baked into it the architecture. we could put in some value to be replaced with the task arch. but there may be other corner cases lurking.

see http://koji.stg.fedoraproject.org/koji/taskinfo?taskID=90070223 as an example task with multiple architectures. and if you go to the arches you will see 'cockpit-0.95-1-x86_64.tar' in the parameters

[twaugh]

Design overtook this.