string.format must be a string according to
https://json-schema.org/understanding-json-schema/reference/string.html#format

We were using our own version of metaschema with a patch for this. Now
we are back to using official schema, but with a custom format
`internet-address` that should be equivalent to:

    "oneOf": [
      {
	"format": "hostname"
      },
      {
	"format": "ipv4"
      },
      {
	"format": "ipv6"
      }
    ],