Delete all allocatable_ips on ip_block delete

* Calls the delete method on the ip generator to clear the block of any
  allocatable_ips so the FK doesn't fail the constraint on block delete
* Fixes LP963396

Change-Id: Iaa1915e79adde23970ad7405477e8d82467a8a58

melange/ipam/models.py

@@ -287,6 +287,7 @@ class IpBlock(ModelBase):
         for block in self.subnets():
             block.delete()
         IpAddress.find_all(ip_block_id=self.id).delete()
+        ipv4.plugin().get_generator(self).delete()
         super(IpBlock, self).delete()
 
     def policy(self):

melange/ipv4/db_based_ip_generator/generator.py

@@ -49,3 +49,6 @@ class DbBasedIpGenerator(object):
     def ip_removed(self, address):
         models.AllocatableIp.create(ip_block_id=self.ip_block.id,
                                     address=address)
+
+    def delete(self):
+        models.AllocatableIp.find_all(ip_block_id=self.ip_block.id).delete()