type this command:
sudo visudo
add this line:
username ALL=NOPASSWD: ALL
Note: When multiple entries match for a user, they are applied in order. Where there are multiple matches, the last match is used (which is not necessarily the most specific match). So add the line above at the bottom of file.
Source:
– http://askubuntu.com/questions/334318/sudoers-file-enable-nopasswd-for-user-all-commands