Introduce non-main package repo's

[seemiller]

What this PR does / why we need it:
We are currently using a single repo, main, for our packages. We need to provide our users with a stable repository of packages while still being able to further develop TCE. This change will introduce non-main repos. Think of these as alpha or beta repos where engineers can make changes to packages and continue development of new features without impacting existing users/clusters.

Which issue(s) this PR fixes:

Fixes #385

Describe testing done for PR:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[seemiller]

So my first take on this is to introduce an "environment" file, such as alpha.yaml. This file will act as the values file for ytt. As you can see in the file below, you can specify values for the repo and specific version information for each plugin. In this case I've declared a new name for a package repo and a URL with a new tag. This repo then has a cert-manager with referencing an alpha version and image tag.

@data/values
 ---

 repo:
   #! The name of the Package Repository
   name: tce-alpha.tanzu.vmware

   #! The URL for the repo image
   url: projects.registry.vmware.com/tce/main:alpha

 cert_manager:
   metadata:
     name: cert-manager.1.1.0-vmware-alpha
   spec:
     version: 1.1.0-vmware-alpha
     image: projects.registry.vmware.com/tce/cert-manager-package:alpha

You would use ytt to transform the existing repo and package manifests to update with values specific to the new "environment" file. This creates a repo with all the version information of your new environment.

ytt --ignore-unknown-comments -f addons/repos/main.yaml -f addons/repos/config/alpha.yaml -f addons/repos/config/overlays -f addons/repos/main/packages 

[joshrosso]

@seemiller Looks great!

I'm wondering about the value of addons/repos/config/alpha.yaml.

The addons/repos/main.yaml would be the "official" declaration of our main repo.
A addons/repos/beta.yaml could then be the "official" declaration of our beta repo.

This would make it easy for users to look in one location for the declaration of our repos.

"Overrides" like addons/repos/config/alpha.yaml are fine for package/repo developers to declare, but I think i'd want declarations of our repos to be flat.

Let me know your thoughts on this.

Some notes:

* if we made the above change, I think we could move `addons/repos/config/overlays` to `addons/repos/overlays`.
* With this change, we should set every package to reference `:latest` and assume in real usage, it'll get rendered via overlay.

[seemiller]

@joshrosso @stmcginnis I've updated some things on this branch. Y'all wanna take a look?

[jorgemoralespou]

Some comments for you @seemiller

[joshrosso]

@seemiller let's not introduce addons/repos/stages with a .gitkeep file. Instead, any relevant make target should create this directory and we should add it to the main .gitignore.