Update the quota usage when instances are shutdown

Update the quota usage when instances are shutdown

Summary
Today when the instances are shutdown, the user cannot reuse the resources on the hypervisor since the quota still use the resources, which restricts from reusing the resources.

Release Date
Post Havana

Rationale
More the flexibility given to user, the better it would be for adoption of Openstack Nova. If we could provide user with the reuse of resources of shutdown instances by providing resource calculation at instance startup & shutdwon it would definitely improve the user experience.

User Stories
As a Openstack End User I want to reuse the resources such as RAM,VCPUs when the instaneces are shutdown

Design
TBD

Code Changes
TBD

Test Plan
TBD

Demo Plan
TBD