user supplied subnet

[mkumatag]

Fixes: #257

User guide:

    "builders": [
        {
            "type": "powervs",
            "instance_name": "packer-build-{{timestamp}}",
            "subnet_id": "a31d66d7-14df-4731-83cb-e8fa02cd5555",   <<<<<<< new filed added
            "key_pair_name": "mkumatag-pub-key",

in the above example, subnet_id is the new filed got added and user need to submit the existing subnet id

[ppc64le-cloud-bot]

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