Allow DNS labels with up to 63 characters

[MartinWeindel]

What would you like to be added:
The length of DNS labels is restricted to 63 characters according to RFC 1035, but currently the DNS controller manager only supports DNS labels up to 55 characters. The reason is the additional DNS TXT record containing the owner meta data of the record. Its domain name is derived from the original domain name by prefixing it with comment- (8 chars). This means this 63 characters limit also applies to the meta data record.
An alternative way of naming the meta data record should be added (e.g. with some hashing part) to overcome this restriction.

Why is this needed:
Support full 63 characters for users.