Allow query of the drained node attribute

This patch exports the drained attribute:
  - LUQueryNodes accepts now the drained field
  - RAPI exports it for node objects
  - gnt-node info shows it now (along newly-added master_candidate and
    offline flags)
  - gnt-node list can list it (but not by default)
  - to the iallocator scripts

Reviewed-by: imsnah