Pre-check all DNS entries before deploying VMs

[jbemmel]

Currently, Metro will first deploy VSD, and then check if the VSC DNS name is correct. If it is not, the deployment fails, leaving the system in an incomplete state.

It would be better to first validate the entire config ( against the schema, and also details like DNS mappings, availability of images and licenses, resource constraints, etc. ) before starting to make changes. That way, users can correct issues earlier, reducing the iterative cycle time ( fail early )

[ghost]

Good idea. We could do the validation only when there's been a change to the deployment. That way, subsequent runs would not unnecessarily pay the price for the validation.