Move "dagster" command to "args" for k8s container object
Instead of explicitly setting a command, we only set args for the k8s container. This is so the user can override the command if needed through their Docker entrypoint.
Reviewers: catherinewu, rexledesma
Reviewed By: catherinewu, rexledesma
Differential Revision: https://dagster.phacility.com/D5452