Update ubuntu_config.md

There is a warning in /etc/resolv.conf (Ubuntu Server 12.04.3) like:
"DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN"

So, /etc/resolv.conf can't be edited. In this case, we should edit /etc/resolvconf/resolv.conf.d/base and after that, we should run "resolvconf -u"