[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/dustin/go-humanize/humanize.go b/vendor/github.com/dustin/go-humanize/humanize.go
new file mode 100644
index 0000000..a2c2da3
--- /dev/null
+++ b/vendor/github.com/dustin/go-humanize/humanize.go
@@ -0,0 +1,8 @@
+/*
+Package humanize converts boring ugly numbers to human-friendly strings and back.
+
+Durations can be turned into strings such as "3 days ago", numbers
+representing sizes like 82854982 into useful strings like, "83 MB" or
+"79 MiB" (whichever you prefer).
+*/
+package humanize