Allocated container metrics per application should be exposed using Yarn & ATS rest APIs

Currently allocatedMB and allocatedVCores are being exposed by the RM and ATS rest APIs.
But we don't have the allocatedContainers exposed per application.

This metric can be exposed as a additional param in the existing rest APIs.

RM: http://<rm http address:port>/ws/v1/cluster/apps/

{appid}
ATS: http(s)://<timeline server http(s) address:port>/ws/v1/applicationhistory/apps/{appid}

This would be essential for application types like TEZ which have container re-use.