[Fixes #3] Manage /etc/resolv.conf as a symlink

This file should be a symlink to the real file that is rendered dynamically
by the resolvconf package. We call out to `dpkg-reconfigure` to ensure that
the target is created correctly, which may not be the case is a machine is
converted to a statically managed `resolv.conf`.

/cc @maxamg @philandstuff @samjsharpe @nickstenning