Prevent ingress deletion when endpoint count == 1
Prevent ingress deletion when endpoint count == 1

We should not delete an ingress network just because its endpoint count
drops to 1 (the IP address of the sandbox).  This addresses a regression
where the ingress sandbox could be deleted on workers when the last
container leave said sandbox.

Signed-off-by: Chris Telfer <9cf1a5ba8f085fe3bf87fd3e4704febafc338899@docker.com>

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