[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/pkg/fileutil/read_dir.go b/vendor/go.etcd.io/etcd/pkg/fileutil/read_dir.go
new file mode 100644
index 0000000..2eeaa89
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/pkg/fileutil/read_dir.go
@@ -0,0 +1,70 @@
+// Copyright 2018 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fileutil
+
+import (
+	"os"
+	"path/filepath"
+	"sort"
+)
+
+// ReadDirOp represents an read-directory operation.
+type ReadDirOp struct {
+	ext string
+}
+
+// ReadDirOption configures archiver operations.
+type ReadDirOption func(*ReadDirOp)
+
+// WithExt filters file names by their extensions.
+// (e.g. WithExt(".wal") to list only WAL files)
+func WithExt(ext string) ReadDirOption {
+	return func(op *ReadDirOp) { op.ext = ext }
+}
+
+func (op *ReadDirOp) applyOpts(opts []ReadDirOption) {
+	for _, opt := range opts {
+		opt(op)
+	}
+}
+
+// ReadDir returns the filenames in the given directory in sorted order.
+func ReadDir(d string, opts ...ReadDirOption) ([]string, error) {
+	op := &ReadDirOp{}
+	op.applyOpts(opts)
+
+	dir, err := os.Open(d)
+	if err != nil {
+		return nil, err
+	}
+	defer dir.Close()
+
+	names, err := dir.Readdirnames(-1)
+	if err != nil {
+		return nil, err
+	}
+	sort.Strings(names)
+
+	if op.ext != "" {
+		tss := make([]string, 0)
+		for _, v := range names {
+			if filepath.Ext(v) == op.ext {
+				tss = append(tss, v)
+			}
+		}
+		names = tss
+	}
+	return names, nil
+}