.circleci/config.yml: vendor gnostic for k8s.io/client-go (DataDog#585)

k8s.io/client-go has github.com/googleapis/gnostic as a dependency.
Recently, gnostic renamed some packages to be consistent with go style (google/gnostic#155)
This change breaks client-go, which will no longer build when clients are checked out with e.g. go get k8s.io/client-go/kubernetes.

This break in k8s.io/client-go is causing our CI to fail. This commit addresses the issue by checking out k8s.io/client-go manually and vendoring the last working version of gnostic in k8s.io/client-go/vendor.

.circleci/config.yml

@@ -105,6 +105,16 @@ jobs:
           https://github.com/go-redis/redis
           contrib/go-redis/redis/vendor/github.com/go-redis/redis
 
+    - run:
+        name: Vendor github.com/googleapis/gnostic for k8s.io/client-go
+        # This step checks out k8s.io/client-go and vendors 
+        # github.com/googleapis/gnostic to fix a breaking change made in
+        # gnostic. See kubernetes/client-go#741
+        # TODO(knusbaum): remove this once the breaking change is resolved or propagated
+        command: |
+          git clone https://github.com/kubernetes/client-go $GOPATH/src/k8s.io/client-go
+          git clone --branch v0.4.0 https://github.com/googleapis/gnostic $GOPATH/src/k8s.io/client-go/vendor/github.com/googleapis/gnostic
+
     - run:
         name: Fetching dependencies
         command: |