[dagster-k8s] Have job image from executor config take precedence instead of erroring
Summary: Instead of throwing an error, have a job image from executor config take precedence over a job image from a user code deployment. Also report an engine event to make it clear that this is happening.
Reviewers: catherinewu, alangenfeld
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D5597