Allow changing the image builder default tag in postsubmit jobs

[triffer]

Description
We need to build two different types of images with a different set of features for the istio-manager. For our postsubmit jobs on the main and release branches we defined an additional tag for each of those images (default image, experimental image).

The problem we are facing is that each of those builds also pushs an image tag with the format of v{{ .Date }}-{{ .ShortSHA }} defined in the kaniko-build-config.yaml.

Building our default image:

Building our image with experimental features:

Reasons
Make the tags created during image build reliable when building multiple tags for one commit.

The consequence is that the v{{ .Date }}-{{ .ShortSHA }} tag is unusable, because it's unreliable as whatever build finishes last will have that tag. We now have the risk that we publish tags that should not be used, but we cannot control this.

Acceptance Criteria
It would be good to have a possibility to override or extend the default tag in a job, so that
all image tags pushed during a postsubmit job can be controlled in the job configuration.

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