Allow configuring a repository to be exempt from renovate management

### Problem

For JSS repositories, we are likely to require customized renovate configurations. There are some aspects of the CFT-managed renovate configuration we may want to retain.

### Proposal

* Allow configuring opt-out of a repository from renovate management by CFT
* Consider moving some of the [template renovate configuration](https://github.com/terraform-google-modules/terraform-google-module-template/blob/master/.github/renovate.json) to [GitHub-hosted presets](https://docs.renovatebot.com/config-presets/#github) so repositories that opt out can easily include custom regexManagers or other configurations that may remain universally useful.