Add make targets to generate manifests locally

[corlettb]

https://www.pivotaltracker.com/n/projects/1275640/stories/185391018

What

Add the ability to generate the manifest files locally.

Make targets:

-- Generate manifests against a fake environment
make cf_manifest_stub
make app_autoscaler_manifest_stub
make prometheus_manifest_stub

-- Generate environment manifests
gds aws paas-<aws env>-admin -- make <env> cf_manifest
gds aws paas-<aws env>-admin -- make <env> app_autoscaler_manifest

-- Prometheus requires credhub credentials
gds aws paas-<aws env>-admin -- make <env> credhub
credhub login --sso
make <env> prometheus_manifest

Why

When doing upstream updates it can be very awkward to test manifest tweaks. These new make targets allow SREs to quickly iterate on manifest tweaks.

How to review

Try the new make targets to generate manifest files.
Look at the changes.

Future

Tickets will be created for the following:

• Adjust the existing pipeline to use these make targets to remove code duplication
• Rescope the secrets the prometheus manifest generation requires so the generation no longer needs access to credhub.

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🚨⚠️ Please do not merge this pull request via the GitHub UI ⚠️🚨

[jackjoy-gds]

LGTM