Allow test_apis.py to kill used port. See #62.

test_apis.py cannot start its demo server so when plugin workers
tried to access the target url, the plugin worker will see
"couldn't reach site" and aborts the plugin (we probably need to fix
that), This commit only fix test_apis.py ``_kill_ports`` because
when the same fix is applied to tests/functional/plugins/base.py all
tests fail to pass. We will leave this issue aside for a while.