[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/reflect2/.gitignore b/vendor/github.com/modern-go/reflect2/.gitignore
new file mode 100644
index 0000000..7b26c94
--- /dev/null
+++ b/vendor/github.com/modern-go/reflect2/.gitignore
@@ -0,0 +1,2 @@
+/vendor
+/coverage.txt