Add warning log message when cloud config is malformed (missing required string).

[ddelnano]

This PR is meant to address #202.

I don't think the XO provider should provide bulletproof validation when there is an official data source for cloudinit, but hopefully this should catch very obvious errors in the cloud config.

Testing

• [x] Existing test suite triggers new logging when running a VM test (all cloud config used in the acceptance tests is malformed and will trigger this new check)

  
  ddelnano@ddelnano-desktop:~/go/src/github.com/ddelnano/terraform-provider-xenorchestra$ TEST=TestAccXenorchestraVm_createWithMutipleDisks TF_LOG=trace make testacc 2>&1 | grep WARNING
  2022/12/12 16:45:35 [WARNING] cloud config does not start with required text `#cloud-config`.
  2022/12/12 16:45:35 [WARNING] Validate that your configuration is well formed according to the documentation (https://cloudinit.readthedocs.io/en/latest/topics/format.html).
  2022/12/12 16:46:54 [WARNING] cloud config does not start with required text `#cloud-config`.
  2022/12/12 16:46:54 [WARNING] Validate that your configuration is well formed according to the documentation (https://cloudinit.readthedocs.io/en/latest/topics/format.html).

[4censord]

If one uses cloud-configs that are GZIP compressed, or in the mime-multipart format, this also logs this warning now.

[ddelnano]

I removed this temporarily to further investigate to see what it would take to detect gzip compressed or mime-multipart formats.