Add priority tiers to support multi-tenancy

Tier is kind of priority level, it will include a type and priority level. The type can be either quota or fair share. The reason that we want to have `Tier` is mainly for defining resources allocations with priority for now. One example is for `Quota`, if we have more quotas than total resources in the cluster, then with the `Tier` logic, we can make sure the high priority tier quota can get allocations first. Also the high priority tier quota can preempt resources from preemptable quota (A new concept for quota and still under discussion). With `Tier`, we can also enable tasks with priority (task priority is based on the resources priority), and high priority tasks can preempt resources from low priority tasks.

Current design document: https://docs.google.com/document/d/1bPHREn1AfUQIAGwZUS7yLFZLW6ycO2WbmEzW6o92NV0/edit