[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/bbolt/bolt_linux.go b/vendor/go.etcd.io/bbolt/bolt_linux.go
new file mode 100644
index 0000000..7707bca
--- /dev/null
+++ b/vendor/go.etcd.io/bbolt/bolt_linux.go
@@ -0,0 +1,10 @@
+package bbolt
+
+import (
+	"syscall"
+)
+
+// fdatasync flushes written data to a file descriptor.
+func fdatasync(db *DB) error {
+	return syscall.Fdatasync(int(db.file.Fd()))
+}