fix(daemon): prepend host /etc/hosts instead of bind mounting

systemd systems do not require a /etc/hosts file exists since an nss
module is shipped that creates localhost implicitly. So, mounting
/etc/hosts can fail on these sorts of systems, as was reported on CoreOS
in issue #5812.

Instead of trying to bind mount just prepend the hosts entries onto the
containers private /etc/hosts. Maybe this will fix #5857 too?

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)