fixing an issue in the debian networking template related to DNS servers.  Since Ubuntu has moved away from /etc/default/networking since the 12.04 release, the build_network_settings doesn't create this file on Ubuntu systems higher than 12.04, the get_network_settings shouldn't return anything including defaults.
Merge pull request #13563 from garethgreenaway/debian_ip_fixes

Fixes to debian_ip.py and friends.