Balloons: enable ignoring the error where a pod has been annotated with a non-existing balloon type

### Description

Under some scenarios the balloon type in the annotation can be only a preference rather than a strict requirement. If a preferred balloon type does not exist, he pod/container should be handled as if there would not be balloon annotation at all. That is, `chooseBalloonDef()` based on namespaces in balloon types or assigning the pod to a default balloon.

Potential ways to introduce such behavior include:
- use an annotation that specifically means preference instead of a requirement
- add an option to ignore the non-existing balloon type error