bump_release: adjust tests for desired behaviour
bump_release: adjust tests for desired behaviour

When a base image is built, only the new-style release label should
be set. This allows for the old-style labels to be phased out as
builds naturally occur.

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