[spike] Configuring alternative registries

[philbrookes]

We need to answer some questions around what having multiple registry options in the dns operator should look like:

• how does the DNS Operator know which registry to use (runtime, policy, etc.)
• how to send configuration information for registries to the DNS Operator (i.e. redis auth details)
• what happens if DNS Operators are configured on the same zone but with different registries (register registries in a TXT record to ensure congruency?)
• Are there any issues we might find, based on our existing assumption of a TXT registry
• What happens when more than 20 controllers run against the same zone when that limit is lifted (e.g. 50 controllers using redis registry)

As DynamoDB is already coded as a registry in externalDNS it might be faster to answer these questions using DynamoDB before moving to #322