Inserting config from JSON skips validation

[Alexander Kilroy]

Hi,

I noticed some strange behaviour when trying to retrieve my keys. I forgotten my JSON base doc had underscores and noticed it did in fact set them, consider this example:

```
CONFIG="$(cat <<EOF
{
  "key_with_underscores": null
}
EOF
)"

echo $CONFIG | xargs -d'\n' -I {} snapctl set toplevelkey={}

# If I go to retrieve this key, it is set with the underscores as:
`toplevelkey.key_with_underscores`
# And is therefor not retrievable
```

By setting JSON into snapd via snapctl, it circumvents the validation, allowing keys with underscores to be set but not retrieved.