Fixes router external_fixed_ip tests sometimes failing with Conflict.

Currently, a Conflict is thrown for tests like
test_create_router_external_fixed_ips because an IP address will
already be in use, due to improper clean up in other tests,
causing this test to throw a Conflict exception.

Change-Id: Iec090d4740c2fd7e230086e3e35e25ef8812ba71
Closes-Bug: #1673290
Partial-Bug: #1670553
1 file changed