Allow external-dns replica count to be set to zero

[josh-ferrell]

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What would you like to be added: The ability to install an external-dns helm release with the replica count set to zero.

Why is this needed: We have failover clusters in secondary regions with hot spare workloads that can not run concurrently with the primary region. external-dns handles the DNS records for each region. Allowing the replica count to be configured as zero will allow external-dns to be deployed to the secondary region and scaled up to 1 only during failover. It will also allow us to scale the primary region to zero and have external-dns handle the DNS changes necessary for failover.