Return stack id in /v3/buildpacks call for CNBs

Previously, this returned the name of the stack custom resource.  The
more consistent identifier is the id of the stack itself.

[#169924422]

Co-authored-by: Chris Selzo <cselzo@pivotal.io>
Co-authored-by: Sannidhi Jalukar <sjalukar@pivotal.io>

Author: selzoc

app/fetchers/kpack_buildpack_list_fetcher.rb

@@ -11,7 +11,7 @@ def fetch_all(message)
       version_map = default_builder.status.builderMetadata.each.with_object({}) do |metadata, h|
         h[metadata.id] = metadata.version
       end
-      stack = default_builder.spec.stack
+      stack = default_builder.status.stack.id
       created_at = Time.parse(default_builder.metadata.creationTimestamp)
 
       latest_condition = default_builder.status.conditions[0]

spec/unit/fetchers/kpack_buildpack_list_fetcher_spec.rb

@@ -42,7 +42,10 @@ module VCAP::CloudController
               status: 'True',
               type: 'Ready',
             }
-          ]
+          ],
+          stack: {
+            id: 'org.cloudfoundry.stacks.cflinuxfs3'
+          }
         }
       )
     }
@@ -69,7 +72,7 @@ module VCAP::CloudController
         expect(buildpack1.name).to(eq('paketo-community/ruby'))
         expect(buildpack1.id).to(eq('paketo-community/ruby@0.0.11'))
         expect(buildpack1.filename).to(eq('paketo-community/ruby@0.0.11'))
-        expect(buildpack1.stack).to(eq(cflinuxfs3_stackname))
+        expect(buildpack1.stack).to(eq('org.cloudfoundry.stacks.cflinuxfs3'))
         expect(buildpack1.guid).to(be_blank)
         expect(buildpack1.state).to(eq('READY'))
         expect(buildpack1.position).to(eq(0))
@@ -83,7 +86,7 @@ module VCAP::CloudController
         expect(buildpack2.name).to(eq('paketo-buildpacks/java'))
         expect(buildpack2.id).to(eq('paketo-buildpacks/java@1.14.0'))
         expect(buildpack2.filename).to(eq('paketo-buildpacks/java@1.14.0'))
-        expect(buildpack2.stack).to(eq(cflinuxfs3_stackname))
+        expect(buildpack2.stack).to(eq('org.cloudfoundry.stacks.cflinuxfs3'))
         expect(buildpack2.guid).to(be_blank)
         expect(buildpack2.state).to(eq('READY'))
         expect(buildpack2.position).to(eq(0))