[Urgent] fix unit test failure casused by IPAM driver
[Urgent] fix unit test failure casused by IPAM driver

1. What is the problem
After recent updates now Neutron uses pluggable IPAM driver backend by default,
while in the unit tests for network functionality in the Tricircle, unpluggable
IPAM driver backend is assumed to be used. So this default backend change causes
most of the unit tests fail.

2. What is the solution to the problem
In the setup of unit tests, set "ipam_driver" option as empty string to use
unpluggable IPAM driver backend.

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

Change-Id: Ifa85d4e209ac0fa04c21db316eee6ebc41981dab

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