Add interfaces for getting limits without enforcing

We currently have a public Enforcer interface for getting limits and
calculating usage but it is not yet possible to retrieve only limits
without calculating usage.

While working on unified limits support in nova, we realized we need a
way to get limits only. In nova there are legacy APIs for showing quota
limits and initially we will provide compat by proxying to keystone to
get the limits.

This adds public interfaces for getting limits to Enforcer.

Related to blueprint unified-limits-nova

Change-Id: I22234e0bb6b3a1cecb29a6b99a3afcd02ffdbf5f