add managed-by label

[ryandawsonuk]

Would like to follow kubernetes recommended labels. Some of those seem to overlap with our existing labels though so that's a bigger topic. Immediately it would be good to just add the managed-by label. That one would mean all pods and services created by the seldon operator could be matched with a single label. That would be useful, for example, for using with the prometheus operator and its PodMonitor and ServiceMonitor concepts.