Add fixed ip cleanup to test_add_fixed_ip

There is a data race on clean up with the test test_add_fixed_ip
because it can currently fail during resource_cleanup [0] with:
"One or more ports have an IP allocation from this subnet."

[0] http://logs.openstack.org/36/575536/4/gate/patrole-admin/ccf48f1/testr_results.html.gz

Change-Id: I71099c62cb9c49739ec21579f24d4fd8a9cbf432
1 file changed