allow /etc/hosts /etc/resolv.conf and /etc/hostname to be changed

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

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
add small doc

Signed-off-by: Victor Vieux <vieux@docker.com>
Merge pull request #5129 from vieux/host_resolv

allow /etc/hosts /etc/resolv.conf and /etc/hostname to be changed