Fair Scheduler: allow default queue placement rule to take an arbitrary queue

Currently 'default' rule in queue placement policy,if applied,puts the app in root.default queue. It would be great if we can make 'default' rule optionally point to a different queue as default queue .
This default queue can be a leaf queue or it can also be an parent queue if the 'default' rule is nested inside nestedUserQueue rule(YARN-1864).