Decoupling instance resource management from bastion module

[kral2]

Bastion module currently creates its own compute resource. In the long run, it should be better to offload that task to a dedicated module that do just that : handling compute instances.

Transitioning toward this model would be a specific project for this module as this will most probably introduce breaking changes to the module API.

I would batch this API change with other more structural changes to the module for a "next major version", similar to what we are planning with vcn module and leave it as it is for now, with a deprecation notice first.

Bastion will probably need to leverage a capable compute module at some point instead of creating the resource directly itself. That can be a good time to change the module API and avoid repetitive changes over a short period of time.

I am going to create an issue about this strategy, and transitioning toward an independent compute module should probably be treated as a project regarding this module.

Originally posted by @kral2 in #19 (comment)

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