QUICKSTART-VMWARE: fix 'tuftool download' instruction

[etungsten]

Issue number:
N/A

Description of changes:

    QUICKSTART-VMWARE: fix 'tuftool download'
    
    There was a breaking change in tuftool v0.7.0 where the output directory
    must not exist when calling 'tuftool download'. This fixes the quickstart
    instructions for downloading an OVA.

Testing done:
I ran the commands in the quickstart with the latest tuftool version (v0.7.1):

$ tuftool download "${OUTDIR}" --target-name "${OVA}" \
>    --root ./root.json \
>    --metadata-url "https://updates.bottlerocket.aws/2020-07-07/${VARIANT}/x86_64/" \
>    --targets-url "https://updates.bottlerocket.aws/targets/"
Downloading targets to "vmware-k8s-1.21-v1.6.1"
        -> bottlerocket-vmware-k8s-1.21-x86_64-v1.6.1.ova

$ govc import.spec "${OUTDIR}/${OVA}" > bottlerocket_spec.json

$ cat bottlerocket_spec.json
{
  "DiskProvisioning": "flat",
  "IPAllocationPolicy": "dhcpPolicy",
  "IPProtocol": "IPv4",
  "NetworkMapping": [
    {
      "Name": "VM Network",
      "Network": ""
    }
  ],
  "MarkAsTemplate": false,
  "PowerOn": false,
  "InjectOvfEnv": false,
  "WaitForIP": false,
  "Name": null
}

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[zmrow]

🏞️