Mock file system when testing duplicate IPs
Mock file system when testing duplicate IPs

Testing the calculation of unique IP addresses does not need to
read/write to the file system in order to provide an accurate
test. Removing the IO saves roughly 50% of the previous test runtimes,
which is helpful for developers iterating on code locally.

Change-Id: I23197390e743b7b5ccb06efd91a71f2d9beaf5c1

cfarquhar/openstack-ansible,cloudnull/os-ansible-deployment,weezer/openstack-ansible,bunchc/openstack-ansible,yanyao/newton,cloudnull/os-ansible-deployment,stackforge/os-ansible-deployment,openstack/openstack-ansible,BjoernT/os-ansible-deployment,BjoernT/openstack-ansible,openstack/openstack-ansible,robb-romans/openstack-ansible,os-cloud/os-ansible-deployment,cfarquhar/openstack-ansible,Logan2211/openstack-ansible,BjoernT/os-ansible-deployment,weezer/openstack-ansible,os-cloud/os-ansible-deployment,stackforge/os-ansible-deployment,Logan2211/openstack-ansible,bunchc/openstack-ansible,yanyao/openstack-ansible,robb-romans/openstack-ansible,yanyao/openstack-ansible,BjoernT/os-ansible-deployment,yanyao/newton,BjoernT/openstack-ansible