[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/go.etcd.io/etcd/etcdserver/api/snap/snappb/snap.proto b/vendor/go.etcd.io/etcd/etcdserver/api/snap/snappb/snap.proto
new file mode 100644
index 0000000..cd3d21d
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/etcdserver/api/snap/snappb/snap.proto
@@ -0,0 +1,14 @@
+syntax = "proto2";
+package snappb;
+
+import "gogoproto/gogo.proto";
+
+option (gogoproto.marshaler_all) = true;
+option (gogoproto.sizer_all) = true;
+option (gogoproto.unmarshaler_all) = true;
+option (gogoproto.goproto_getters_all) = false;
+
+message snapshot {
+	optional uint32 crc  = 1 [(gogoproto.nullable) = false];
+	optional bytes data  = 2;
+}