Write something that makes it easy to validate a yaml file

[mw44118]

A common source of (minor) bugs is when we forget to add an entry to a prod.yaml file.

It would be fun it the configwrapper code could verify that all keys exist in a yaml file, like:

$ xyz-config --verify scratch.yaml

and then all the properties would be accessed.
Maybe instead of using code like

@property
def google_maps_API_key(self):

We could define a thing like required_entry that does the exact same thing, but then also, registers this property as one to be verified.