Ensure that external network don't have any ports before deletion

In module
neutron_tempest_plugin.api.admin.test_external_network_extension
we need to ensure that there is no any leftover ports, like e.g.
floatingip_agent_gateway port before network deletion.

Closes-bug: #1875344

Change-Id: I8226e999d9ec8e521b39ab915aaa503425174987
1 file changed