[VOL-3981] Voltha Core restarts if it can't subscribe to Kafka

This commit fixes the following issues:
1) It creates a loop to try to resubscribe to kafka on error.  This
is an issue that occurs randomly, especially when the kafka broker
is up and running but not truly ready to create a new topic.

2) Fixes an issue where the event proxy start was incorrect setting
the cluster messaging bus probe to out of service and never sets it
to running.   This was causing the Core to wait forever for the
probe to be ready.

Change-Id: Idf22481f85e4b576440301f2859da7ddf2d8c688
diff --git a/go.mod b/go.mod
index 18e8af4..56b4125 100644
--- a/go.mod
+++ b/go.mod
@@ -3,6 +3,7 @@
 go 1.16
 require (
+	github.com/cenkalti/backoff/v3 v3.2.2
 	github.com/gogo/protobuf v1.3.2
 	github.com/golang/mock v1.5.0
 	github.com/golang/protobuf v1.3.2