Fix delete quota of subprojects

Fix delete quota of subprojects

This patch aims to fix the bug in which cinder raises
an error when performing a delete quota operation in
a subproject.

Steps to reproduce:
- Create a project A in keystone (e.g openstack project create A);
- Create a project B in keystone with A as its parent (e.g openstack
  project create B --property parent_id=<id of project A>);
- Update the quota value of A (e.g. openstack quota set
  <id of project A> --volume 5)
- Update the quota value of B (e.g. openstack quota set
  <id of project B> --volume 2)
- Try to delete the quota of B (e.g. cinder quota-delete
  <id of project B>)

Change-Id: Id1a749433c3679f1b2adf87ea0a280366c5e4a15
Closes-bug: #1496058