Refactoring REST API re-design changes in handlers.go, handlers_test.go

Tasks:

1. Avoid repeating "Partition not found", "Queue not found" etc asserts.
2. Make use of utils.GetNormalizedPartitionName to avoid cluster id in partition name
3. Address https://github.com/apache/incubator-yunikorn-core/pull/224#discussion_r636531149