Don't override container's `entrypoint`

[dchambers]

Bring the Kubernetes executor into line with the Docker executor, which never overrides entrypoint, but only cmd. In Kubernetes this is done using the args parameter instead of the command parameter.

[dchambers]

FYI: I've not been able to test this change at present. I created an automated build on Docker Hub, but this seems to grab pre-built binaries from S3 and there doesn't seem to be a CI configured to build these binaries or instructions for doing it manually.

If somebody gives me a steer I should be able to test that this change works in practice.