interfaces_by_fixed_ip no extra port

test_create_list_show_delete_interfaces_by_fixed_ip was allowed
to make one extra port/interface creation optional
in case the CONF.compute.fixed_network_name was defined,
but in this case the test failed at teardown time (double port delete).
Turned out the test case normally not even deletes the fixed_ip port,
inside the test case.

There is no point for having 3 interface in this test, so just
adding/deleting the fixed_ip interface
and removing th 3th `extra` interface part from the test.

The extra port was problematic in (multiple) shared network case,
however fixed_ip interface creation part
has the right logic to use an explicit network.

Related-Bug: #1790864
Change-Id: I6e3e53c4ac26b4fef09fefb9c590dfa91f577565
1 file changed