Remove neutronclient mocks from network exceptions tests
Remove neutronclient mocks from network exceptions tests

Neutronclient mock is replaced by mocking REST calls using
base.RequestsMockTestCase class

Change-Id: I488de8613a2e2580f49533b319603d159f7dc832

openstack/python-openstacksdk,dtroyer/python-openstacksdk,openstack/python-openstacksdk,stackforge/python-openstacksdk,openstack-infra/shade,dtroyer/python-openstacksdk,openstack-infra/shade,stackforge/python-openstacksdk