Load controller config from configMap

[mikenairn]

What

Allow the dns operator to load controller configuration options from a configMap that exists in the same namespace as the controller. The configMap should allow env vars that correspond to controller flags to be set and override the value.

Why

Allowing controller configuration to be set this way will make it easier to create different deployment scenarios, especially when configuration of the dns operator needs to be controlled from the kuadrant operator level.

Main reason for this at the moment is to support the idea of a dns operator "mode" that will be introduced as part of the "Cluster Aware DNSRecord Delegation" work.

How

• Add configMap to dns operator deployment (no values)
• Update the dns operator to load controller options from env vars if they exist, but default back to flag value
• Add documentation