Support per-container cpu / memory bursting.

Currently, the cgroup cpu policy is burned in at the agent level. The user can start the agent with --cgroups_enable_cfs to apply cfs quota to all containers (effectively disallowing exceeding the requested amount of cpus for all containers on the agent). The agent does not allow containers to exceed the requested memory (except when a container's requested memory is shrunk).

We should instead enable per-container cpu / memory bursting via per-container cpu and memory requests / limits.

See kubernetes for an example of a per container cpu/memory bursting API:

https://kubernetes.io/docs/concepts/configuration/manage-compute-resources-container/#resource-requests-and-limits-of-pod-and-container