Opportunistic containers : Combined patch for branch-2

This is a combined patch targeting branch-2 of the following JIRAs which have already been committed to trunk :

YARN-5938. Refactoring OpportunisticContainerAllocator to use SchedulerRequestKey instead of Priority and other misc fixes
YARN-5646. Add documentation and update config parameter names for scheduling of OPPORTUNISTIC containers.
YARN-5982. Simplify opportunistic container parameters and metrics.
YARN-5918. Handle Opportunistic scheduling allocate request failure when NM is lost.
YARN-4597. Introduce ContainerScheduler and a SCHEDULED state to NodeManager container lifecycle.
YARN-5823. Update NMTokens in case of requests with only opportunistic containers.
YARN-5377. Fix TestQueuingContainerManager.testKillMultipleOpportunisticContainers.
YARN-2995. Enhance UI to show cluster resource utilization of various container Execution types.
YARN-5799. Fix Opportunistic Allocation to set the correct value of Node Http Address.
YARN-5486. Update OpportunisticContainerAllocatorAMService::allocate method to handle OPPORTUNISTIC container requests.