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 copy the hosts entries onto the
containers private /etc/hosts.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
integration-cli: tests for /etc/hosts and net=host

Some basic tests to make sure this is acting correctly on machines.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
integration-cli: fix spelling error in test

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Merge pull request #5859 from philips/append-etc-hosts-not-bind

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