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
Merge "Not permitted to delete pools which are used by flavor"