One annoying thing about the set-siteconfig management command is that

it would error out if you tried to set a key that wasn't already

explicitly set, even if a default was defined.
This change fixes that to allow setting those. We still do validation on

the "parent" keys when a setting path is drilling down into dicts, but

any leaf key can now be set.

Saw that I could now set items in siteconfig that I couldn't before.