[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/modern-go/concurrent/executor.go b/vendor/github.com/modern-go/concurrent/executor.go
new file mode 100644
index 0000000..623dba1
--- /dev/null
+++ b/vendor/github.com/modern-go/concurrent/executor.go
@@ -0,0 +1,14 @@
+package concurrent
+
+import "context"
+
+// Executor replace go keyword to start a new goroutine
+// the goroutine should cancel itself if the context passed in has been cancelled
+// the goroutine started by the executor, is owned by the executor
+// we can cancel all executors owned by the executor just by stop the executor itself
+// however Executor interface does not Stop method, the one starting and owning executor
+// should use the concrete type of executor, instead of this interface.
+type Executor interface {
+	// Go starts a new goroutine controlled by the context
+	Go(handler func(ctx context.Context))
+}