Allow replica count to be set to zero

[josh-ferrell]

Description
Allows the replica count for external-dns deployments to be configurable as zero. external-dns helm installs can be done in secondary regions with zero replicas waiting to be scaled up during a failover event. See #4783

The helm chart values.yaml was commented to reflect that external-dns does not support more than one replica.

Checklist

• Unit tests updated
• End user documentation updated

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[ivankatliarchuk]

You may need to add some validation as well. Not sure if external DNS supports HA mode werry well. Having 2+ replicas may throttle DNS API as at the moment there is no leader election.

[ivankatliarchuk]

The problem is described here a bit #2430. Had a DNS outage once due to this behaviour.

[k8s-ci-robot]

PR needs rebase.

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