[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/github.com/json-iterator/go/any_nil.go b/vendor/github.com/json-iterator/go/any_nil.go
new file mode 100644
index 0000000..d04cb54
--- /dev/null
+++ b/vendor/github.com/json-iterator/go/any_nil.go
@@ -0,0 +1,69 @@
+package jsoniter
+
+type nilAny struct {
+	baseAny
+}
+
+func (any *nilAny) LastError() error {
+	return nil
+}
+
+func (any *nilAny) ValueType() ValueType {
+	return NilValue
+}
+
+func (any *nilAny) MustBeValid() Any {
+	return any
+}
+
+func (any *nilAny) ToBool() bool {
+	return false
+}
+
+func (any *nilAny) ToInt() int {
+	return 0
+}
+
+func (any *nilAny) ToInt32() int32 {
+	return 0
+}
+
+func (any *nilAny) ToInt64() int64 {
+	return 0
+}
+
+func (any *nilAny) ToUint() uint {
+	return 0
+}
+
+func (any *nilAny) ToUint32() uint32 {
+	return 0
+}
+
+func (any *nilAny) ToUint64() uint64 {
+	return 0
+}
+
+func (any *nilAny) ToFloat32() float32 {
+	return 0
+}
+
+func (any *nilAny) ToFloat64() float64 {
+	return 0
+}
+
+func (any *nilAny) ToString() string {
+	return ""
+}
+
+func (any *nilAny) WriteTo(stream *Stream) {
+	stream.WriteNil()
+}
+
+func (any *nilAny) Parse() *Iterator {
+	return nil
+}
+
+func (any *nilAny) GetInterface() interface{} {
+	return nil
+}