Warn about challenge name on chal create

> The Challenge "a_challenge" is invalid: metadata.name: Invalid value: "a_challenge": a lowercase RFC 1123 subdomain must consist of lower case alphanumeric characters, '-' or '.', and must start and end with an alphanumeric character (e.g. 'example.com'

Ideally, kctf should warn about this on `kctf chal create` instead of when attempting to start the challenge.