Preserve usage and reservations on quota deletion

Preserve usage and reservations on quota deletion

Current API deletes quota usage and reservations on quota limit
deletion.

According to API documentation what should only happen is that quotas
limits revert to default values by deleting tenant/user limits.

This patch fixes this issue.

APIImpact: Delete on os-quota-sets will no longer remove usage and
           reservation quotas. Those quotas are handled by Cinder
           service.
UpgradeImpact: There is no upgrade impact afaik.
Closes-Bug: #1410034
Change-Id: I9340b6f78623cfa5b505886ad75b8e4d3cd6131b