fix kubeproxy some tests

* NAT46: fix test cleanup, missing del keyword
* NAT66: fix kube-proxy vip, is ipv6
* add some missing kp_put_writer_lock
* wipe flowtable after each unit test
  * Add new cli api: "test kube-proxy flowtable flush" to flushes everything
  * Call this new cli function after the end of each kube-proxy unit test.
  * same as commit b3d1b203579226ca5136b9d6a2744577d07cfcc6 for the lb plugin

Change-Id: I4146f44841328ec96eb66729e3bae3d40f33e4aa
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>