[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/errors.go b/vendor/go.etcd.io/etcd/etcdserver/errors.go
new file mode 100644
index 0000000..d0fe289
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/etcdserver/errors.go
@@ -0,0 +1,51 @@
+// Copyright 2015 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 etcdserver
+
+import (
+	"errors"
+	"fmt"
+)
+
+var (
+	ErrUnknownMethod              = errors.New("etcdserver: unknown method")
+	ErrStopped                    = errors.New("etcdserver: server stopped")
+	ErrCanceled                   = errors.New("etcdserver: request cancelled")
+	ErrTimeout                    = errors.New("etcdserver: request timed out")
+	ErrTimeoutDueToLeaderFail     = errors.New("etcdserver: request timed out, possibly due to previous leader failure")
+	ErrTimeoutDueToConnectionLost = errors.New("etcdserver: request timed out, possibly due to connection lost")
+	ErrTimeoutLeaderTransfer      = errors.New("etcdserver: request timed out, leader transfer took too long")
+	ErrLeaderChanged              = errors.New("etcdserver: leader changed")
+	ErrNotEnoughStartedMembers    = errors.New("etcdserver: re-configuration failed due to not enough started members")
+	ErrLearnerNotReady            = errors.New("etcdserver: can only promote a learner member which is in sync with leader")
+	ErrNoLeader                   = errors.New("etcdserver: no leader")
+	ErrNotLeader                  = errors.New("etcdserver: not leader")
+	ErrRequestTooLarge            = errors.New("etcdserver: request is too large")
+	ErrNoSpace                    = errors.New("etcdserver: no space")
+	ErrTooManyRequests            = errors.New("etcdserver: too many requests")
+	ErrUnhealthy                  = errors.New("etcdserver: unhealthy cluster")
+	ErrKeyNotFound                = errors.New("etcdserver: key not found")
+	ErrCorrupt                    = errors.New("etcdserver: corrupt cluster")
+	ErrBadLeaderTransferee        = errors.New("etcdserver: bad leader transferee")
+)
+
+type DiscoveryError struct {
+	Op  string
+	Err error
+}
+
+func (e DiscoveryError) Error() string {
+	return fmt.Sprintf("failed to %s discovery cluster (%v)", e.Op, e.Err)
+}