Allow control of managed-by label through use of environment variable

[stack72]

Related: #1357

This allows the label to be controlled in a simple way for now

▶ PULUMI_KUBERNETES_MANAGED_BY_LABEL=mycompany pulumi up
Previewing update (dev)
     Type                             Name                      Plan
 +   pulumi:pulumi:Stack              testing-new-workflow-dev  create
 +   └─ kubernetes:core/v1:Namespace  nginx                     create
Resources:
    + 2 to create
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+ pulumi:pulumi:Stack: (create)
    [urn=urn:pulumi:dev::testing-new-workflow::pulumi:pulumi:Stack::testing-new-workflow-dev]
    + kubernetes:core/v1:Namespace: (create)
        [urn=urn:pulumi:dev::testing-new-workflow::kubernetes:core/v1:Namespace::nginx]
        [provider=urn:pulumi:dev::testing-new-workflow::pulumi:providers:kubernetes::default_2_8_0::04da6b54-80e4-46f7-96ec-b56ff0331ba9]
        apiVersion: "v1"
        kind      : "Namespace"
        metadata  : {
            labels: {
                app.kubernetes.io/managed-by: "mycompany"
            }
            name  : "testing"
        }

Proposed changes

Related issues (optional)