ASCN-250 / ASCN-307: Sandbox move + Discovery GCP deployment targets during CNAB build instead of hard-coding

[stephen-vakil]

As we go through another refactor of infrastructure, knowing clusters and other details might change in the future, and knowing we eventually need to install in multiple clusters, we need to be able to discover when running our CNAB deploy which deployment targets (clusters) and deployment groups (projects) we will deploy to, instead of hard-coding them as variables.

Additionally, having to copy/paste all this stuff that doesn't change everywhere is a bit of toil that can be solved by centralize as much as possible into the base image.

This PR therefore does the following:

• Use an updated cnab base image that includes most of our script/logic for deployment
• Anything app-specific is now in run.ps1

See this PR for more details on how the base image scripts are organized:
https://github.com/StackEng/base-docker-images/pull/59

How to test

• test locally by running the Invoke-CNAB script until the octopus changes are in place too.
• test in octopus using this pr branch

[WouterDeKort]

I left some small questions/comments but overall I think this looks good!