Add test for cleanupServiceDiscovery
Add test for cleanupServiceDiscovery

Unit test for the cleanupServiceDiscovery,
follow up of PR: #1985

Signed-off-by: Flavio Crisciani <59eae1aa696b1b39cd87a38bdba3f0b2630647f9@docker.com>

mahak/libnetwork,abhi/libnetwork,kolyshkin/libnetwork,docker/libnetwork,fcrisciani/libnetwork,fcrisciani/libnetwork,mahak/libnetwork,abhi/libnetwork,kolyshkin/libnetwork,vdemeester/libnetwork,medallia/libnetwork,docker/libnetwork,medallia/libnetwork,docker/libnetwork,medallia/libnetwork,vdemeester/libnetwork,mahak/libnetwork,vdemeester/libnetwork,kolyshkin/libnetwork,abhi/libnetwork