[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/xiang90/probing/server.go b/vendor/github.com/xiang90/probing/server.go
new file mode 100644
index 0000000..0e7b797
--- /dev/null
+++ b/vendor/github.com/xiang90/probing/server.go
@@ -0,0 +1,25 @@
+package probing
+
+import (
+	"encoding/json"
+	"net/http"
+	"time"
+)
+
+func NewHandler() http.Handler {
+	return &httpHealth{}
+}
+
+type httpHealth struct {
+}
+
+type Health struct {
+	OK  bool
+	Now time.Time
+}
+
+func (h *httpHealth) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	health := Health{OK: true, Now: time.Now()}
+	e := json.NewEncoder(w)
+	e.Encode(health)
+}