Support cpu-memory combination on container/pod

Implements something like AWS EKS Fargate Pod Configuration: https://docs.aws.amazon.com/eks/latest/userguide/fargate-pod-configuration.html

Basically, we still want to allow users to specify "arbitrary" cpu and memory, but Zun should round up to a cpu-memory combination. The round-up cpu-memory combination is used in scheduling, resource claiming, placement reporting, etc.