Initialise and source configuration from a ConfigMap

## WHY

Part of the CodeFlare operator re-design, as described in https://github.com/project-codeflare/adr/pull/9.

Follows up #273.

## WHAT

The structured configuration should be sourced from a ConfigMap. If it doesn't exist already, the operator should initialise it with sensible defaults when it starts.

To be done on top of #216.