Raise error when NUM_NODES is set to a value other than 0 or 1

Since IMPALA-4746 is not implemented there is basically two modes of Impala to execute queries:

• NUM_NODES = 0
  • create a distributed plan and execute the query on as many executors as possible
• NUM_NODES = 1
  • create a single node plan and execute the query on the coordinator

But currently users can specify any value to NUM_NODES, e.g. 42, but the behavior they get would correspond to the NUM_NODES = 0 case (distributed mode).

Impala should raise an error when NUM_NODES is set to other value than 0 or 1 so users won't have wrong assumptions.