Honor user-typed worker command

[ywskycn]

The worker command was hardcoded as sleep 365d, no matter what command users specify. This PR is to honor users' inputs.

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This change is 

[ywskycn]

Rethinking this patch's approach. Another way would be reusing spec.mpiReplicaSpecs.Worker.template.spec.containers[0].command|args.

[terrytangyuan]

Yes, especially now that v1alpha2 is more flexible, we can just reuse the worker template spec.

[ywskycn]

Yes, especially now that v1alpha2 is more flexible, we can just reuse the worker template spec.

Update the PR which directly checks if users place a customized worker command in the spec.

[terrytangyuan]

Thanks!

/lgtm
/approve

[k8s-ci-robot]

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