fix docker info with lxc 1.0.0

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
remove WARNING writter twice

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #4376 from vieux/fix_lxc_version

fix docker info with lxc 1.0.0
Merge pull request #4379 from vieux/remove_warnings_twice

remove WARNING writter twice