[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/grpc-gateway/internal/stream_chunk.proto b/vendor/github.com/grpc-ecosystem/grpc-gateway/internal/stream_chunk.proto
new file mode 100644
index 0000000..55f42ce
--- /dev/null
+++ b/vendor/github.com/grpc-ecosystem/grpc-gateway/internal/stream_chunk.proto
@@ -0,0 +1,15 @@
+syntax = "proto3";
+package grpc.gateway.runtime;
+option go_package = "internal";
+
+import "google/protobuf/any.proto";
+
+// StreamError is a response type which is returned when
+// streaming rpc returns an error.
+message StreamError {
+	int32 grpc_code = 1;
+	int32 http_code = 2;
+	string message = 3;
+	string http_status = 4;
+	repeated google.protobuf.Any details = 5;
+}