pubsys: template AMI names

[tjkirch]

Per #1091, it would be nice if users could specify in Infra.toml a template of desired AMI names.

Currently there's a template baked into Makefile.toml called PUBLISH_AMI_NAME_DEFAULT that users can override using cargo make ami -e PUBLISH_AMI_NAME=foo (with #1091), but if you have a consistent naming pattern, keeping a template in Infra.toml is simpler.

[stmcginnis]

We may need something like this for out of tree builds, but that effort will address that if/when needed. Since we don't need a separate issue specifically for that part of it, going to close this as not something we've found we've strongly needed otherwise.