Pulumi shouldn't fetch the latest AMI by default

When using pulumi update with an EKS cluster, pulumi’s default behavior is to always fetch the latest version of the EKS AMI if nodeAmiId isn’t specified.

While this behavior can be convenient, unexpected upgrades to the EKS AMI can cause unexpected downtime or break the cluster entirely.

For production situations, it’s strongly recommended that you explicitly pass the nodeAmiId parameter so that you have full control over when your nodes upgrade.

pulumi-eks’s default behavior should enforce best practices. I propose the following:

1. Make nodeAmiId a required parameter (force users to make a conscious decision about the AMI that they want to use).
2. Provide the existing fetch behavior only as a convenience function for users that don’t need fine control over when their cluster upgrades.
3. Warn people in the documentation for the above helper function about not using this in production.

See https://pulumi-community.slack.com/archives/C84L4E3N1/p1553791785005000 for more discussion.

[metral]

@gitfool The new AWS SSM Parameter Store API was just announced yesterday. #258 is tracking the work to leverage it.

[gitfool]

Falling back to requiring AMI Id is not so friendly. It would be better to be able to specify an exact k8s version, e.g. 1.14.6, or an exact AMI name, e.g. amazon-eks-node-1.14-v20190906, and still have the AMI Id looked up, since the optimal AMI Id will vary depending on region and so should match the region the Pulumi stack is in.