During cluster deletion, magnum tries to delete the cluster's load
balancers in advance of deleting the heat stack. If these load balancers
do not exist for some reason, the cluster deletion will fail with an
error such as the following:
Failed to pre-delete resources for cluster
748b628a-2cd8-456f-8aee-c93804b2099b, error: list indices must be
integers or slices, not str.
This happens because the heat stack has the physical_resource_id set to
None for the load balancer, which causes the load_balancer_show method
of octavia client to GET all load balancers, rather than just one. The
returned data is a list, rather than a dict, leading to the error above.
This change fixes the issue by checking if physical_resource_id is set
to None, and skipping the load balancer deletion if so.