[Fullstack] Handle properly BrokenPipeError exception
[Fullstack] Handle properly BrokenPipeError exception

NetcatTester class should handle BrokenPipeError exception
and not raise it to fail test immediately if nc process wasn't
yet started when it tries first time to read/write something to
it.

Change-Id: Ica953cc2038b24c4b3985447b393763912aa6abd
Closes-Bug: #1871908

openstack/neutron,mahak/neutron,openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron