[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/srv/srv.go b/vendor/go.etcd.io/etcd/pkg/srv/srv.go
new file mode 100644
index 0000000..c356002
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/pkg/srv/srv.go
@@ -0,0 +1,142 @@
+// 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 srv looks up DNS SRV records.
+package srv
+
+import (
+	"fmt"
+	"net"
+	"net/url"
+	"strings"
+
+	"go.etcd.io/etcd/pkg/types"
+)
+
+var (
+	// indirection for testing
+	lookupSRV      = net.LookupSRV // net.DefaultResolver.LookupSRV when ctxs don't conflict
+	resolveTCPAddr = net.ResolveTCPAddr
+)
+
+// GetCluster gets the cluster information via DNS discovery.
+// Also sees each entry as a separate instance.
+func GetCluster(serviceScheme, service, name, dns string, apurls types.URLs) ([]string, error) {
+	tempName := int(0)
+	tcp2ap := make(map[string]url.URL)
+
+	// First, resolve the apurls
+	for _, url := range apurls {
+		tcpAddr, err := resolveTCPAddr("tcp", url.Host)
+		if err != nil {
+			return nil, err
+		}
+		tcp2ap[tcpAddr.String()] = url
+	}
+
+	stringParts := []string{}
+	updateNodeMap := func(service, scheme string) error {
+		_, addrs, err := lookupSRV(service, "tcp", dns)
+		if err != nil {
+			return err
+		}
+		for _, srv := range addrs {
+			port := fmt.Sprintf("%d", srv.Port)
+			host := net.JoinHostPort(srv.Target, port)
+			tcpAddr, terr := resolveTCPAddr("tcp", host)
+			if terr != nil {
+				err = terr
+				continue
+			}
+			n := ""
+			url, ok := tcp2ap[tcpAddr.String()]
+			if ok {
+				n = name
+			}
+			if n == "" {
+				n = fmt.Sprintf("%d", tempName)
+				tempName++
+			}
+			// SRV records have a trailing dot but URL shouldn't.
+			shortHost := strings.TrimSuffix(srv.Target, ".")
+			urlHost := net.JoinHostPort(shortHost, port)
+			if ok && url.Scheme != scheme {
+				err = fmt.Errorf("bootstrap at %s from DNS for %s has scheme mismatch with expected peer %s", scheme+"://"+urlHost, service, url.String())
+			} else {
+				stringParts = append(stringParts, fmt.Sprintf("%s=%s://%s", n, scheme, urlHost))
+			}
+		}
+		if len(stringParts) == 0 {
+			return err
+		}
+		return nil
+	}
+
+	err := updateNodeMap(service, serviceScheme)
+	if err != nil {
+		return nil, fmt.Errorf("error querying DNS SRV records for _%s %s", service, err)
+	}
+	return stringParts, nil
+}
+
+type SRVClients struct {
+	Endpoints []string
+	SRVs      []*net.SRV
+}
+
+// GetClient looks up the client endpoints for a service and domain.
+func GetClient(service, domain string, serviceName string) (*SRVClients, error) {
+	var urls []*url.URL
+	var srvs []*net.SRV
+
+	updateURLs := func(service, scheme string) error {
+		_, addrs, err := lookupSRV(service, "tcp", domain)
+		if err != nil {
+			return err
+		}
+		for _, srv := range addrs {
+			urls = append(urls, &url.URL{
+				Scheme: scheme,
+				Host:   net.JoinHostPort(srv.Target, fmt.Sprintf("%d", srv.Port)),
+			})
+		}
+		srvs = append(srvs, addrs...)
+		return nil
+	}
+
+	errHTTPS := updateURLs(GetSRVService(service, serviceName, "https"), "https")
+	errHTTP := updateURLs(GetSRVService(service, serviceName, "http"), "http")
+
+	if errHTTPS != nil && errHTTP != nil {
+		return nil, fmt.Errorf("dns lookup errors: %s and %s", errHTTPS, errHTTP)
+	}
+
+	endpoints := make([]string, len(urls))
+	for i := range urls {
+		endpoints[i] = urls[i].String()
+	}
+	return &SRVClients{Endpoints: endpoints, SRVs: srvs}, nil
+}
+
+// GetSRVService generates a SRV service including an optional suffix.
+func GetSRVService(service, serviceName string, scheme string) (SRVService string) {
+	if scheme == "https" {
+		service = fmt.Sprintf("%s-ssl", service)
+	}
+
+	if serviceName != "" {
+		return fmt.Sprintf("%s-%s", service, serviceName)
+	}
+	return service
+}