[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/lease/leasepb/lease.proto b/vendor/go.etcd.io/etcd/lease/leasepb/lease.proto
new file mode 100644
index 0000000..1169d9f
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/lease/leasepb/lease.proto
@@ -0,0 +1,25 @@
+syntax = "proto3";
+package leasepb;
+
+import "gogoproto/gogo.proto";
+import "etcd/etcdserver/etcdserverpb/rpc.proto";
+
+option (gogoproto.marshaler_all) = true;
+option (gogoproto.sizer_all) = true;
+option (gogoproto.unmarshaler_all) = true;
+option (gogoproto.goproto_getters_all) = false;
+option (gogoproto.goproto_enum_prefix_all) = false;
+
+message Lease {
+  int64 ID = 1;
+  int64 TTL = 2;
+  int64 RemainingTTL = 3;
+}
+
+message LeaseInternalRequest {
+  etcdserverpb.LeaseTimeToLiveRequest LeaseTimeToLiveRequest = 1;
+}
+
+message LeaseInternalResponse {
+  etcdserverpb.LeaseTimeToLiveResponse LeaseTimeToLiveResponse = 1;
+}