Add KUBECTL_NODE_SHELL_IMAGE_PULL_SECRET_NAME to customize imagePullSecrets

[huandu]

Add a new KUBECTL_NODE_SHELL_IMAGE_PULL_SECRET_NAME environment variable to customize imagePullSecrets.

For instance, if KUBECTL_NODE_SHELL_IMAGE_PULL_SECRET_NAME is set to my-secret, imagePullSecrets in Pod spec will be "imagePullSecrets": [ { "name": "my-secret" } ].