Keep the top-level resource stable within a major version

[mikhailshilkov]

Design and implement the mechanism to keep the top-level resources stable within a major version.

According to our plan in pulumi/pulumi-azure-nextgen#169 (comment):

At 1.0, for a given resource, we will likely pick an API version that is the latest. If a later version is published by Microsoft and it contains breaking changes, we will NOT introduce this version to the top-level resource. It will be available under a versioned namespace only.

One obvious way is to stop updating provider_list.json but let's spend some time considering pros/cons and other options.