test/k8sT: specify image tag for Kafka proxy image
test/k8sT: specify image tag for Kafka proxy image

While the only tag available on DockerHub is 'latest' for
'docker.io/spotify/kafkaproxy', explicitly state that the tag is 'latest', and
add a comment indicating that the only tagged version on DockerHub is 'latest'.

Signed-off by: Ian Vernon <ian@cilium.io>

scanf/cilium,tklauser/cilium,scanf/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,scanf/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,scanf/cilium,tgraf/cilium,cilium-team/cilium,cilium/cilium,cilium/cilium,scanf/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,scanf/cilium,cilium-team/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,scanf/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium