Feature: ObjectTemplate variables

[erikgb]

After my POC on dynamic review environments, I encountered unavoidable duplications in my ObjectTemplate:

---
apiVersion: templates.kluctl.io/v1alpha1
kind: ObjectTemplate
metadata:
  name: stas-tenant-review
spec:
  serviceAccountName: gitops-reconciler
  prune: true
  matrix:
    - name: mr
      object:
        ref:
          apiVersion: templates.kluctl.io/v1alpha1
          kind: ListGitlabMergeRequests
          name: stas-tenant
        jsonPath: status.mergeRequests
        expandLists: true
  templates:
    - object:
        apiVersion: accurate.cybozu.com/v2
        kind: SubNamespace
        metadata:
          name: "stas-rev-{{ matrix.mr.source_branch | slugify }}"
    - object:
        apiVersion: source.toolkit.fluxcd.io/v1
        kind: GitRepository
        metadata:
          name: "stas-rev-{{ matrix.mr.source_branch | slugify }}"
        spec:
          interval: 1m0s
          url: https://gitlab.statnett.no/stas/stas-tenant.git
          ref:
            branch: "{{ matrix.mr.source_branch }}"
    - object:
        apiVersion: kustomize.toolkit.fluxcd.io/v1
        kind: Kustomization
        metadata:
          name: "stas-rev-{{ matrix.mr.source_branch | slugify }}"
        spec:
          force: false
          interval: 60m0s
          path: ./review
          prune: true
          retryInterval: 2m0s
          sourceRef:
            kind: GitRepository
            name: "stas-rev-{{ matrix.mr.source_branch | slugify }}"
          targetNamespace: "stas-rev-{{ matrix.mr.source_branch | slugify }}"
          timeout: 3m0s
          wait: true

While this works, I had to repeat the Jinja expression for review namespace multiple times. I wonder if we could/should add a new feature to extract variables that the templates could use. With this example, it could be as simple as:

---
apiVersion: templates.kluctl.io/v1alpha1
kind: ObjectTemplate
metadata:
  name: stas-tenant-review
spec:
  serviceAccountName: gitops-reconciler
  prune: true
  matrix:
    - name: mr
      object:
        ref:
          apiVersion: templates.kluctl.io/v1alpha1
          kind: ListGitlabMergeRequests
          name: stas-tenant
        jsonPath: status.mergeRequests
        expandLists: true
  vars:
    branch: "{{ matrix.mr.source_branch }}"
    namespace:  "stas-rev-{{ matrix.mr.source_branch | slugify }}"   
  templates:

This will make the templates easier to read, reduce the risk of minor errors/typos (Jinjia is hard), and allow for easier patching of OTs with e.g. Kustomize.