Overlay driver to check for chain presence
Overlay driver to check for chain presence

- When creating and programming the global overlay chain,
  gracefully handle the case where the chain already exists.
  Today the driver logs an Error and does not attempt to insert
  the return rule if the chain is already present.

Signed-off-by: Alessandro Boch <5bd2eb5b6f60921d9a2b9cb6a0a86bb947f3d361@docker.com>

kolyshkin/libnetwork,YujiOshima/libnetwork,vdemeester/libnetwork,cpuguy83/libnetwork,sainath14/libnetwork,mavenugo/libnetwork,mYmNeo/libnetwork,docker/libnetwork,mbanikazemi/libnetwork,medallia/libnetwork,msabansal/libnetwork,ityangchen/libnetwork,tmakita/libnetwork,mahak/libnetwork,jimmycmh/libnetwork,cpuguy83/libnetwork,mahak/libnetwork,medallia/libnetwork,ityangchen/libnetwork,allencloud/libnetwork,jmzwcn/libnetwork,coolljt0725/libnetwork,aboch/libnetwork_new,medallia/libnetwork,abhi/libnetwork,fcrisciani/libnetwork,docker/libnetwork,WeiZhang555/libnetwork,mYmNeo/libnetwork,fcrisciani/libnetwork,mbanikazemi/libnetwork,coolljt0725/libnetwork,abhi/libnetwork,aboch/libnetwork_new,curtiszimmerman/libnetwork,WeiZhang555/libnetwork,jimmycmh/libnetwork,kolyshkin/libnetwork,amitkris/libnetwork,abhi/libnetwork,mahak/libnetwork,YujiOshima/libnetwork,jmzwcn/libnetwork,kolyshkin/libnetwork,docker/libnetwork,tmakita/libnetwork,msabansal/libnetwork,amitkris/libnetwork,curtiszimmerman/libnetwork,mavenugo/libnetwork,sainath14/libnetwork,vdemeester/libnetwork,vdemeester/libnetwork,pyakpyak/libnetwork,allencloud/libnetwork,pyakpyak/libnetwork