Kafka Trigger: cannot accept multiple brokers

[spaghettifunk]

Describe the bug
The same issue was present in the EventSource and fixed in #894 . However, the same problem appears in the Sensor code (https://github.com/argoproj/argo-events/blob/master/sensors/triggers/kafka/kafka.go#L89)

To Reproduce
Steps to reproduce the behavior:

1. Create an EventSource to receive a message
2. Create a Source with the KafkaTrigger
3. Set under the url a string with multiple kafka brokers url separated by a comma
4. Send a message
5. See error in the sensor pod

Expected behavior
The expected behaviour should be that you can define multiple brokers URL in the url paramater separated by a comma

Environment (please complete the following information):

• Kubernetes: v1.18.1
• Argo: v2.13.2
• Argo Events: v1.2.0

Additional context
The error you get in the sensor pod is the following:

{"level":"error","ts":1609935890.4802158,"logger":"argo-events.sensor","caller":"sensors/trigger.go:80","msg":"failed to invoke the trigger","sensorName":"vod-ratings-sensor","trigger":"kafka-ratings","error":"kafka: client has run out of available brokers to talk to (Is your cluster reachable?)","stacktrace":"github.com/argoproj/argo-events/sensors.(*SensorContext).GetTrigger\n\t/home/runner/work/argo-events/argo-events/sensors/trigger.go:80\ngithub.com/argoproj/argo-events/sensors.(*SensorContext).triggerActions\n\t/home/runner/work/argo-events/argo-events/sensors/listener.go:234\ngithub.com/argoproj/argo-events/sensors.(*SensorContext).ListenEvents.func1.3\n\t/home/runner/work/argo-events/argo-events/sensors/listener.go:148"}

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