Disable authentication prompts in 15.04?
As I understand it, these prompts use Polkit, not sudo, for authorization. Therefore, setting NOPASSWD in sudoers won't make any difference to it. To make Polkit skip password prompts, create a .pkla file in /etc/polkit-1/localauthority/50-local.d/ (say /etc/polkit-1/localauthority/50-local.d/99-nopassword.pkla) containing:
[No password prompt]
Identity=unix-group:sudo
Action=*
ResultActive=yes
See the pklocalauthority manpage for more information.