Move "dagster" command to "args" for k8s container object

Summary:
Resolves https://github.com/dagster-io/dagster/issues/3081

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.

Test Plan:
integration
x

Reviewers: catherinewu, rexledesma

Reviewed By: catherinewu, rexledesma

Differential Revision: https://dagster.phacility.com/D5452