update/remove flavor selection

[planthaber]

The flavor selection still states a compatibility from ubuntu 14.04, whci is afaik not true anymore.

https://github.com/rock-core/package_set/blob/master/init.rb#L65-L81

As we don't have stable releases anymore, I wonder if the flavor system is used somewhere else or if we could remove the option.

Or at least silently default to master (remove the question) and just keep the option to set ROCK_SELECTED_FLAVOR in the init.rb?

Otherwise we skould at least update the ubuntu version mentions in the question to 18.04