Currently the API allows an operator to delete a flavor even when it
is in use by a load balancer. This patch corrects this by blocking
the deletion of a flavor that is currently in use by a load balancer.
It also correctly handles load balancers in the "DELETED"
provisioning_status.
Change-Id: Ie6d4f74e36c2fb7cee4e0ff1e198602c5d8394cc
Story: 2006782
Task: 37307