[VOL-2193] Create mocks for Kafka Client and Etcd

This commit consists of:
1) A kafka client mock that implements the kafka client interface
under voltha-lib-go/pkg/kafka/client.go
2) An embedded Etcd server that runs in-process and represents an
Etcd server.

Change-Id: I52a36132568e08c596bb4136918bebcb654a3b99
diff --git a/vendor/github.com/json-iterator/go/go.mod b/vendor/github.com/json-iterator/go/go.mod
new file mode 100644
index 0000000..e05c42f
--- /dev/null
+++ b/vendor/github.com/json-iterator/go/go.mod
@@ -0,0 +1,11 @@
+module github.com/json-iterator/go
+
+go 1.12
+
+require (
+	github.com/davecgh/go-spew v1.1.1
+	github.com/google/gofuzz v1.0.0
+	github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421
+	github.com/modern-go/reflect2 v0.0.0-20180701023420-4b7aa43c6742
+	github.com/stretchr/testify v1.3.0
+)