MINOR: cli: implement wait srv-unknown

MINOR: cli: implement wait srv-unknown

Define a new CLI wait condition named "srv-unknown".

This will report if the server name is currently not used by an instance
in a specific backend. An error is immediately reported if a matching
instance is found, unless it is flagged for background deletion.

Typically, this is useful after CLI "purge server" to detect the
deletion completion and before creating a new instance via "add server".