Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/github.com/grpc-ecosystem/grpc-gateway/runtime/marshal_httpbodyproto.go

@@ -0,0 +1,43 @@
+package runtime
+
+import (
+	"google.golang.org/genproto/googleapis/api/httpbody"
+)
+
+// SetHTTPBodyMarshaler overwrite the default marshaler with the HTTPBodyMarshaler
+func SetHTTPBodyMarshaler(serveMux *ServeMux) {
+	serveMux.marshalers.mimeMap[MIMEWildcard] = &HTTPBodyMarshaler{
+		Marshaler: &JSONPb{OrigName: true},
+	}
+}
+
+// HTTPBodyMarshaler is a Marshaler which supports marshaling of a
+// google.api.HttpBody message as the full response body if it is
+// the actual message used as the response. If not, then this will
+// simply fallback to the Marshaler specified as its default Marshaler.
+type HTTPBodyMarshaler struct {
+	Marshaler
+}
+
+// ContentType implementation to keep backwards compatability with marshal interface
+func (h *HTTPBodyMarshaler) ContentType() string {
+	return h.ContentTypeFromMessage(nil)
+}
+
+// ContentTypeFromMessage in case v is a google.api.HttpBody message it returns
+// its specified content type otherwise fall back to the default Marshaler.
+func (h *HTTPBodyMarshaler) ContentTypeFromMessage(v interface{}) string {
+	if httpBody, ok := v.(*httpbody.HttpBody); ok {
+		return httpBody.GetContentType()
+	}
+	return h.Marshaler.ContentType()
+}
+
+// Marshal marshals "v" by returning the body bytes if v is a
+// google.api.HttpBody message, otherwise it falls back to the default Marshaler.
+func (h *HTTPBodyMarshaler) Marshal(v interface{}) ([]byte, error) {
+	if httpBody, ok := v.(*httpbody.HttpBody); ok {
+		return httpBody.Data, nil
+	}
+	return h.Marshaler.Marshal(v)
+}