nailgun api sometimes may not clean assigned networks on interfaces

Detailed bug description:
Test https://github.com/openstack/fuel-qa/blob/stable/8.0/fuelweb_test/tests/test_bond_offloading.py#L157 fails sometimes with error: http://paste.openstack.org/show/495323/
After exploring error snapshot we see that after update networks on interface eth0 fuelweb_admin network not deleted, but in update json fuelweb_admin network has only on bond interface
part of nailgun api log:
http://paste.openstack.org/show/495324/
Steps to reproduce:
Please see steps of test:
https://github.com/openstack/fuel-qa/blob/stable/8.0/fuelweb_test/tests/test_bond_offloading.py#L157-L262
Expected results:
 test passed
Actual result:
test failed
Reproducibility:
sometimes