Allow --dashboards-dir flag to deploy.py accept multiple values

[consideRatio]

I'd like to be able to specify multiple --dashboards-dir to deploy both dashboards and global-dashboards folders in one deploy.py CLI call.

Related code can be found below, where I think we can allow for the same flag to be specified multiple times like helm install --values a.yaml --values b.yaml is possible for example. I believe argparse can help us here as well. Maybe action="append" or something like that if I recall correctly.

    parser.add_argument(
        '--dashboards-dir',
        default="dashboards",
        help='Directory of jsonnet dashboards to deploy',
    )

    # ...

    for dashboard in glob(f'{args.dashboards_dir}/*.jsonnet'):