AIAP: Document the requirements for the infrastructure builder image

[ian-howell]

Problem description (if applicable)
Airship-in-a-Pod uses the infra-builder container, which sets up 3 VMs (one used as the ephemeral node, one as a control plane node, and one as a worker node). While this works great for basic testing, we would like to be able to specify different architectures for different needs (e.g. 3 control planes and 2 workers).

AIAP is currently configured such that the task containers are decoupled, i.e. there is nothing stopping a user from swapping out the infra-builder image with their own image. Doing so allows the above desired behavior, however there is currently no documentation on the requirements for the infra-builder container.

Proposed change
Update the AIAP documentation to include the requirements for the infra-builder. This should include pre-reqs (such as required services) as well as outputs (VMs). Furthermore, this should include basic instructions for copying and modifying the default infra-builder image.

This is likely dependent on #648, since fixing it will simplify creating multi-node containers

[pallavgupta]

I think this issue is dependent on issue #652

[gtsteffaniak]

Could you assign to me?

[gtsteffaniak]

PS: https://review.opendev.org/c/airship/airshipctl/+/824200/

[gtsteffaniak]

@eak13 This PS is still active and needs reviews, but I have moved away from the feature team working on this. Someone else may need to take ownership for merging.