[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/matttproud/golang_protobuf_extensions/NOTICE b/vendor/github.com/matttproud/golang_protobuf_extensions/NOTICE
new file mode 100644
index 0000000..5d8cb5b
--- /dev/null
+++ b/vendor/github.com/matttproud/golang_protobuf_extensions/NOTICE
@@ -0,0 +1 @@
+Copyright 2012 Matt T. Proud (matt.proud@gmail.com)