fix(kafka-plugin): Set bootstrapServers and Topics as arrays

[navidshaikh]

Fixes #https://github.com/knative/client-contrib/issues/85

Changes:

• Set bootstrapServers and topics as arrays (still takes a single value on CLI though)
• Bump the eventing-contrib at v0.17.1
• Fix the sink describe logic for describe command
• Fix the e2e test tag

[dsimansk]

I think you need to bump CRD as well https://github.com/knative/client-contrib/blob/master/plugins/source-kafka/test/e2e-tests.sh#L45

Ad error:

 installing 'kn' plugin 'kn-source-kafka' from path '/home/prow/go/src/knative.dev/client-contrib/plugins/source-kafka' to config path: /root/.config/kn
    source_kafka_test.go:76: test kn-source-kafka create source-name
    result_collector.go:75: ERROR: cannot create KafkaSource 'mykafka1' in namespace 'kne2etests0' because: admission webhook "defaulting.webhook.kafka.sources.knative.dev" denied the request: mutation failed: cannot decode incoming new object: json: cannot unmarshal array into Go struct field KafkaSourceSpec.spec.bootstrapServers of type string 

[knative-prow-robot]

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This pull-request has been approved by: daisy-ycguo, navidshaikh

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[daisy-ycguo]

/lgtm