Allow asking for any variant on an ab_test

If ab config changes variant configuration for an ab test,
ab lib used to throw errors if `ab_test.some_variant?` code
was invoked. Now it returns false.

Java/Objective-C implementations seem to behave in similar way.

cc @mmozuras 
Mobile people implemented the lib in a `if ab.is_this_flag_enabled?(:test_name, :flag_name)` kind of way, so they should not have `unless ab.test.off?` kind of issues.
I could still update ab config service to keep all the variants if You think that is a better solution?