Allow DNS top level domain in RFC 1035 check

Issue

I have just used the domain name "suse" - but probably you can use any top level domain name to reproduce this issue.. So when using this top level domain name I get this error:

Using domain name "suse" causes api-group to show error "Error for shared domain name suse: name can contain multiple subdomains, each having only alphanumeric characters and hyphens of up to 63 characters, see RFC 1035."

BUT - "suse" is a valid root domain name - so I do not understand why this check blocks this and why it tells me it is against RFC - IMO a top level domain

Context

In the meantime using top level domain names internal and external is allowed and possible so we should also adjust the check, here.

Steps to Reproduce

Deploy using a top level domain name like "suse", "joedoe" or similar.

Expected result

With top level domain name I would not expect an error and the system should work.

Current result

Error message:

Using domain name "suse" causes api-group to show error "Error for shared domain name suse: name can contain multiple subdomains, each having only alphanumeric characters and hyphens of up to 63 characters, see RFC 1035."

No file or test is named in the issue. Start by locating the RFC 1035 domain-name validation used by the shared-domain API path and reproduce it with a single-label name such as "suse"; done means valid top-level names no longer trigger this error while invalid names remain rejected.