Fix the dhcp clean up for Ubuntu 12.04 or newer's postinstall.sh

For more info, please see the Launchpad issue at https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/910108
Merge pull request #428 from joneslee85/fix-precise-postinstall-dhcp

Fix the dhcp clean up for Ubuntu 12.04 or newer's postinstall.sh