SDK: fix label check for ContainerOP entities

[solovyevt]

Fix ContainerOp label check during compilation step. Previously it was failing with KeyError: 'add-pod-env' if you added custom labels to ContainerOp object with ContainerOp.add_pod_label() function.

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This change is 

[k8s-ci-robot]

Hi @solovyevt. Thanks for your PR.

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[Ark-kun]

Good catch.
/lgtm
/approve

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[Ark-kun]

It can be just op.get('pod_labels', None) and op.pod_labels.get('add-pod-env'], None == 'true')

[Ark-kun]

BTW, what's your use case for labels?

[solovyevt]

@Ark-kun
Got a special use-case with PodAntiAffinity:

container_op.add_pod_label(
        name="io.avoidme",
        value="true"
    )
container_op.add_affinity(affinity=k8s_client.V1Affinity(
        pod_anti_affinity=k8s_client.V1PodAntiAffinity(
            required_during_scheduling_ignored_during_execution=[
                k8s_client.V1PodAffinityTerm(
                    topology_key="kubernetes.io/hostname",
                    label_selector=k8s_client.V1LabelSelector(
                        match_expressions=[
                            k8s_client.V1LabelSelectorRequirement(
                                key="io.avoidme",
                                operator="In",
                                values=["true"]
                            )
                        ]
                    )
                )
            ]
        )
))