[rfe] allow test targets to depend on built images

[mjudeikis]

Ou test jobs are depending on multiple targets to be completed. In example:

    name: pull-ci-openshift-openshift-azure-master-e2e-upgrade-v1.2
    rerun_command: /test e2e-upgrade-v1.2
    spec:
      containers:
      - args:
        - --artifact-dir=$(ARTIFACTS)
        - --give-pr-author-access-to-namespace=true
        - --target=[images]
        - --target=[output:stable:azure-controllers]
        - --target=[output:stable:etcdbackup]
        - --target=[output:stable:metricsbridge]
        - --target=[output:stable:sync]
        - --target=rbac
        - --target=e2e-upgrade-v1.2

Upgrade job e2e-upgrade-v1.2 job is building OSA cluster and testing upgrade process from stable code to testing code. In this example, cluster creation requires images to be available and images have to be available to be pulled from outside (hence rbac template to create a policy to open images for temporary namespace)

        - --target=[output:stable:azure-controllers]
        - --target=[output:stable:etcdbackup]
        - --target=[output:stable:metricsbridge]
        - --target=[output:stable:sync]
        - --target=rbac

In this case, we need to be able to specify that the target e2e-upgrade-v1.2 has a hard dependency on multiple targets to be completed before.

Related: https://github.com/openshift/ci-operator-prowgen/issues/79

/cc @petr-muller @stevekuznetsov @droslean