Don't allow pools deletion if the pool is being used by some flavor

[Flavio Percoco]

It's currently possible to delete a pool even when it's being used by a flavor. An error should be raised if the user tries to do so.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/117706

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/117706
Committed: https://git.openstack.org/cgit/openstack/zaqar/commit/?id=97d05f229c3b5c019da10650aed00fc03868a22e
Submitter: Jenkins
Branch: master

commit 97d05f229c3b5c019da10650aed00fc03868a22e
Author: pengfei wang <email address hidden>
Date: Fri Aug 29 14:56:25 2014 +0800

    Not permitted to delete pools which are used by flavor

    Pool will be used when creating a flavor, in this case
    use is not permitted to delete the pool except after
    removing the relationship either by patching the flavor
    to change the pool it used or deleting the flavor.

    DocsImpact
    Pool deletion can now fail with 403 if the pool is used by flavor

    Change-Id: I8db78a0f55602a865f98eecb0035b139f1ab18b1
    Closes-Bug: #1362174