Add replica config

[matzew]

Fixes #863

Proposed Changes

• adding config for replica, instead hard coded to 1

Release Note

Kafka Channels can configure their replication factor with the `ReplicationFactor` arg.

[matzew]

@nak3 mind to take a look ?

/assign nak3

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In response to this:

@nak3 mind to take a look ?

/assign nak3

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The following is the coverage report on pkg/.
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File	Old Coverage	New Coverage	Delta
contrib/kafka/pkg/controller/channel/reconcile.go	76.6%	76.9%	0.3

[nak3]

@matzew LGTM
(I wondered why it was hard coded, but it looks like there was the plan to update #468 (comment) and seems no side effect for this change.)

[vaikas]

/lgtm
/approve

Are there any documentation changes that should be done for this also?

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: matzew, vaikas-google

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