chore(kontemplate): Prepare kontemplate for depot-merge

This merge will not yet include moving over to buildGo.nix, as support
for testing and such is not present in that library yet.

ops/kontemplate/context/testdata/parent-variable-override.yaml

@@ -0,0 +1,10 @@
+---
+context: k8s.prod.mydomain.com
+include:
+  - name: parent
+    values:
+      foo: bar
+    include:
+      - name: child
+        values:
+          foo: newvalue