[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/grpc-ecosystem/grpc-gateway/runtime/marshaler.go b/vendor/github.com/grpc-ecosystem/grpc-gateway/runtime/marshaler.go
new file mode 100644
index 0000000..98fe6e8
--- /dev/null
+++ b/vendor/github.com/grpc-ecosystem/grpc-gateway/runtime/marshaler.go
@@ -0,0 +1,48 @@
+package runtime
+
+import (
+	"io"
+)
+
+// Marshaler defines a conversion between byte sequence and gRPC payloads / fields.
+type Marshaler interface {
+	// Marshal marshals "v" into byte sequence.
+	Marshal(v interface{}) ([]byte, error)
+	// Unmarshal unmarshals "data" into "v".
+	// "v" must be a pointer value.
+	Unmarshal(data []byte, v interface{}) error
+	// NewDecoder returns a Decoder which reads byte sequence from "r".
+	NewDecoder(r io.Reader) Decoder
+	// NewEncoder returns an Encoder which writes bytes sequence into "w".
+	NewEncoder(w io.Writer) Encoder
+	// ContentType returns the Content-Type which this marshaler is responsible for.
+	ContentType() string
+}
+
+// Decoder decodes a byte sequence
+type Decoder interface {
+	Decode(v interface{}) error
+}
+
+// Encoder encodes gRPC payloads / fields into byte sequence.
+type Encoder interface {
+	Encode(v interface{}) error
+}
+
+// DecoderFunc adapts an decoder function into Decoder.
+type DecoderFunc func(v interface{}) error
+
+// Decode delegates invocations to the underlying function itself.
+func (f DecoderFunc) Decode(v interface{}) error { return f(v) }
+
+// EncoderFunc adapts an encoder function into Encoder
+type EncoderFunc func(v interface{}) error
+
+// Encode delegates invocations to the underlying function itself.
+func (f EncoderFunc) Encode(v interface{}) error { return f(v) }
+
+// Delimited defines the streaming delimiter.
+type Delimited interface {
+	// Delimiter returns the record seperator for the stream.
+	Delimiter() []byte
+}