[RFE] Use `config/settings*` for configuring "secrets", but not "configs"

Discussion spawned from https://github.com/ManageIQ/miq_bot/pull/485#issuecomment-598452344

A note on my previous confusion

I want to clear up that part of this issue was opened up based on a incorrect assumption about how included_repos/excluded_repos worked:

https://github.com/ManageIQ/miq_bot/blob/ca52db9/README.md#enabling-and-disabling-workers

Which I was under the understanding is that this was used to disable what repos are monitored by the bot overall, not what repos are monitored by specific workers in a multi-worker-type environment. I think my points about this still are valid, but wanted to mention this prior in case this wasn't clear to anyone else.

Rational

There are items in the config/settings* that are worth being made public for viability for contributors that don't have server access, specifically the newly added labels: section.

Suggestion

Create a new configuration setting config_repo: that accepts a git URL that can contain one or many yaml files that can be merged into the existing Settings object.

This way, it allows public discussion when something is a configuration issue for the bot for things that don't need to remain secret. We could add it as part of ManageIQ/guides, or create a new repo, for example miq_bot_config.

In addition, it might not hurt have some seed files in that repo for the bot to handle what Repo objects are being added, and it would allow other users to suggest what repos should be watched or at least be aware of what currently being monitored by the bot.