Add support to read AI Platform Notebooks metadata environment

[Gonzalo Gasca Meza]

As part of AI Platform Notebooks DLVM OS, when a user saves a Notebook (.ipynb) file, metadata is written to define image version/container.
Provide ability of tensorflow-cloud to infer Metadata when passing container information if available to AI Platform.

{
    "environment": {
        "name": "tf2-2-3-gpu.2-3.m56",
        "type": "gcloud",
        "uri": "gcr.io/deeplearning-platform-release/tf2-2-3-gpu.2-3:m56"
    },
    "kernelspec": {
        "display_name": "Python 3",
        "language": "python",
        "name": "python3"
    },
    "language_info": {
        "codemirror_mode": {
            "name": "ipython",
            "version": 3
        },
        "file_extension": ".py",
        "mimetype": "text/x-python",
        "name": "python",
        "nbconvert_exporter": "python",
        "pygments_lexer": "ipython3",
        "version": "3.7.8"
    }
}