Allow replica count to be set to zero

[josh-ferrell]

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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
• [X] End user documentation updated