Allow extra lines in /etc/hosts

This adds a --add-host host:ip flag which appends lines to /etc/hosts.  This is needed in places where you want the container to get a different name resolution than it would through DNS.  This was submitted before as #5525, closed, and now I am re-opening.  It has come up 2 or 3 times in the last couple days.

Signed-off-by: Tim Hockin <thockin@google.com>
One thing.  I'm not sure this should go in the config where it could be committed to an image.  I think this probably belongs in the HostConfig.  I'm pretty sure these will end up being non-portable options that we don't want commit and pushed to a public index.