Add Yaml Validation Feature

[krkeegan]

Breaking change

1. Cerberus is now a required package. If you are running from raw source code without installing, you will need to run pip3 install -r requirements.txt again.
2. There is a potential (rather unlikely), that your current yaml configuration file has an error you have not realized. If that is the case, InsteonMQTT will fail to start after upgrading. It will output a clear message describing the error in your yaml file that needs to be fixed.

Proposed change

Adds code to validate the configuration yaml against an included schema on startup. It will also validate the the scenes yaml against an included schema if scenes are enabled.

If validation fails it will print a nice clear message to the user and exit the program.

While a harsh result, it will prevent configuration errors may become latent errors that are hard to diagnose.

The validation is moderately permissive. That is, I have allowed unknown keys to be present in a few places where they may exist in old configuration files.

I believe, for 99.9% of users, if your installation is working currently. The validation will pass. However, there may be some odd situations that are being tolerated by the code that may cause a validation error.

Additional information

• This PR fixes or closes issue: fixes Configuration File Validator with Schema #391

Checklist

• The code change is tested and works locally.
• Local tests pass.
• Tests have been added to verify that the new code works. (I am going to add some example bad yaml files that contain what I suspect are common mistakes).
• Code documentation was added where necessary
• User Documentation added/updated (Still need to add some light user documentation)

[krkeegan]

I should also note that the schema was successfully run against the config-example.yaml and scenes.yaml files as well. So our recommended configuration formats are valid. When a configuration setting has multiple formats, we generally have all examples of these formats in these files, so this is a good test that the schema is tolerating all permitted variations.