ddclient: T5612: Adjust validator and completion for ddclient

Adjust the validator and completion for ddclient to remove unsupported
or superfluous protocols.

Specifically,

• remove 'nsupdate' protocol from the list because there is a separate config path for that protocol (rfc2136)
• remove 'cloudns' protocol from the list because it has non standard configuration and is not supported by our configurator at this time