[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/grpc-ecosystem/go-grpc-middleware/Gopkg.toml b/vendor/github.com/grpc-ecosystem/go-grpc-middleware/Gopkg.toml
new file mode 100644
index 0000000..0a7d4c1
--- /dev/null
+++ b/vendor/github.com/grpc-ecosystem/go-grpc-middleware/Gopkg.toml
@@ -0,0 +1,35 @@
+[[constraint]]
+  name = "github.com/gogo/protobuf"
+  version = "0.5.0"
+
+[[constraint]]
+  branch = "master"
+  name = "github.com/golang/protobuf"
+
+[[constraint]]
+  name = "github.com/opentracing/opentracing-go"
+  version = "1.0.2"
+
+[[constraint]]
+  name = "github.com/sirupsen/logrus"
+  version = "1.0.3"
+
+[[constraint]]
+  name = "github.com/stretchr/testify"
+  version = "1.1.4"
+
+[[constraint]]
+  name = "go.uber.org/zap"
+  version = "1.7.1"
+
+[[constraint]]
+  branch = "master"
+  name = "golang.org/x/net"
+
+[[constraint]]
+  branch = "master"
+  name = "golang.org/x/oauth2"
+
+[[constraint]]
+  name = "google.golang.org/grpc"
+  version = "1.8.0"