generate hostname

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
fix hostname build

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #15 from vieux/generate_hostname

Generate hostname for host networking and fix hostname generation in all cases