Cloud: automate VM size selection

[jefflill]

It would be nice to be able to automate the selection of specific VM sizes when deploying a cluster.

The issue here is that AWS and Azure each have a large number of available VM sizes and it's not always easy to choose which ones you want for a cluster. This can be a problem because VM sizes aren't always available in all regions and it's especially bad for Azure, since users will probably also need to request quotas for VMs.

The basic idea is to express the VM size via parameters like:

• architecture: amd64, arm64 (eventually)
• performance: slow, medium, fast,...
• memory: >= 4GB <= 8GB,...
• cores: 4

Then, we'd query the VM sizes available in the target region and select one that satisfies these constraints.

related: #1671

For Azure (at least), we'd want to select from VMs where the user has available quotas.