Move test_stringified_ips to InstanceTestCase.

There is test_stringified_ips for instance_*
methods in old mixed DbApiTestCase.
This patch moves this test to InstanceTestCase.

blueprint db-api-tests

Change-Id: I771ccec9f41480c8aa90e18b60cc8d75137027d3
Merge "Move test_stringified_ips to InstanceTestCase."