The zuul_copy_output variable is designed to be able to be used as
a zuul job variable so that zuul dictionary merging will work. However,
it's currently being set in the playbook rather than as a job variable,
so it's not possible to supplement it in a child job.
Move it to be a job variable. Also remove the wrapping {} as they should
not be needed to make zuul_copy_output a dictionary.
Change-Id: I78c7fed47c2ab868384c74dbff7904d33d510dd9
a904caded4