Comment 3 for bug 1543094

RetryRequest was raised in 'rollback' block of allocate_ips_for_port_and_store and since rollback failed orignal error got missed.
Created patchest with expanded debug [2] to uncover original failure.

[1] https://github.com/openstack/neutron/blob/master/neutron/db/ipam_pluggable_backend.py#L177
[2] https://review.openstack.org/#/c/285936/