Verify config file structure when Butler is starting

[mountaindude]

The YAML config file has become quite large and can be a challenge to configure correctly if you are not used to it.

The template config file can/should be used when starting out with a new Butler installation and then things are easier.

But when upgrading Butler some config settings may have been added or changed in the new Butler version.
Getting the config file to work can be challenging.

It would be a useful if Butler checked the structure of the config file when starting, making sure that it has the correct contents.
The check would only focus on the structure of the YAML file, not the actual values.
I.e. making sure that all YAML entities are there.

If some setting is missing a log message will be written and Butler stopped.

This will make it easier to find problems caused by incorrect config files.

[mountaindude]

Delivered in v9.0