Replacing vertexExecution in ScheduledUnit with executionVertexID

ScheduledUnit#vertexExecution is nullable but ProgrammedSlotProvider requires it to be non-null to work. This makes ProgrammedSlotProvider not able to be used by new scheduler tests since vertexExecution is never set in the new scheduler code path. It blocks us from reworking tests which are based legacy scheduling to base on the new scheduler.

Besides that, there are 2 other problems caused by the nullable vertexExecution:
1. The log printed in SchedulerImpl#allocateSlotInternal(...) may contain no useful info since the vertexExecution can be null.
2. NPE issue reported in FLINK-16145.

Thus I would propose to replace the nullable vertexExecution with a non-null executionVertexID.