[Urgent] mock _allocate_ips_for_port in unit tests
[Urgent] mock _allocate_ips_for_port in unit tests

1. What is the problem
To simulate Neutron database operations in unit tests, FakeQuery and
FakeSession are defined to replace the original Query and Session
objects. After service subnets feature is merged into Neutron[1], during
ip allocation, Neutron calls Query.add_entity to query subnet by service
type. However, FakeQuery object doesn't have add_entity method, thus
unit tests fail to pass.

2. What is the solution to the problem
Mock _allocate_ips_for_port to bypass the service subnet query, which
is not necessary in the unit tests.

3. What the features need to be implemented to the Tricircle
   to realize the solution
No new feature introduced.

[1] https://review.openstack.org/#/c/350613/

Change-Id: Ieec7f5ab7b63cba0b1c9569e87dd894dd34b07c2

stackforge/tricircle,openstack/tricircle,openstack/tricircle,stackforge/tricircle