[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/soheilhy/cmux/CONTRIBUTORS b/vendor/github.com/soheilhy/cmux/CONTRIBUTORS
new file mode 100644
index 0000000..49878f2
--- /dev/null
+++ b/vendor/github.com/soheilhy/cmux/CONTRIBUTORS
@@ -0,0 +1,12 @@
+# The list of people who have contributed code to the cmux repository.
+#
+# Auto-generated with:
+#		git log --oneline --pretty=format:'%an <%aE>' | sort -u
+#
+Andreas Jaekle <andreas@jaekle.net>
+Dmitri Shuralyov <shurcooL@gmail.com>
+Ethan Mosbaugh <emosbaugh@gmail.com>
+Soheil Hassas Yeganeh <soheil.h.y@gmail.com>
+Soheil Hassas Yeganeh <soheil@cs.toronto.edu>
+Tamir Duberstein <tamir@cockroachlabs.com>
+Tamir Duberstein <tamird@gmail.com>