Make add_remove_fixed_ip tests executable in Nova gate

The tests of test_add_remove_fixed_ip were introduced in
  I8448de16d2243bd08db50562eb2714e07b62ea63
  I7f8dcead3c42ae01354891d9fe6fd1ced0136b72
But they are not executed in Nova gate now. We should test these APIs
like others.

Change-Id: Ifc02080ddf403cb51aaa48b4235c0d286082d10f
Closes-Bug:#1294912
Merge "Make add_remove_fixed_ip tests executable in Nova gate"