koji_promote: use text file for test-case metadata output
koji_promote: use text file for test-case metadata output

This avoids an exception when using Python 3.

DBuildService/atomic-reactor,jarodwilson/atomic-reactor,projectatomic/atomic-reactor,jpopelka/atomic-reactor,jarodwilson/atomic-reactor,DBuildService/atomic-reactor,maxamillion/atomic-reactor,vrutkovs/atomic-reactor,maxamillion/atomic-reactor,fr34k8/atomic-reactor,vrutkovs/atomic-reactor,jpopelka/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor