Description of problem:

Support for keystone unified limits in nova
for centralized quota and resource limits.

The work on unified limtis has been ongoing for several cycles.
https://review.opendev.org/#/c/602201/
to move that effort forword requires a multi cycle investment

While much work has been done to simplify how quotas are implemented in
Nova, there are still some major usability issues for operators with
the current system:

• We don't have consistent support for limit/quota hierarchy across OpenStack
• Requiring operators to set limits individually in each service
  (i.e. Cinder, Nova, Neutron, etc)
• Nova's existing quotas don't work well with Ironic
• No support for custom Resource Class quotas (includes "per flavor" quotas)
• Confusion when API defined quota limits override any changes made to the
  configuration
• Some Nova quotas are unrelated to resource consumption, causing confusion

Transitioning to use Keystone's unified limits, via oslo.limit, will help fix
these issues.

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