Feature: render templates only once

[EtienneBruines]

Proposal

Add a once option to the template block, to indicate the template should only be rendered once and no updates should be performed.

Use-cases

New SSL certificates for haproxy can be added using the Runtime API. This means the original files (that Nomad rendered when starting the allocation) are useful, but updates to those files are not used.

Those updates can be ignored by using change_mode=noop, but that just wastes resources.

Attempted Solutions

Not using the template block at all.

[tgross]

Hi @EtienneBruines! This is a good idea!

Leaving some notes on implementation here. CT already supports a once mode but for Nomad we'd need to plumb that through and then let the template runner exit without blowing up the whole task. We'd also need to document how this would work with task restarts -- most likely we'd end up re-running the template on task restart, but I'd have to spend a little time looking at the task runner hooks to be sure.

I'm going to mark this for roadmapping but also as a "help wanted" item. It might be a good one for one of our more ambitious open source contributors to get their hands on.

[ncode]

This would simplify a lot my current environment