Sudo keeps asking for password, despite that NOPASSWD already set
The group entry %admin  ALL=(ALL) ALL seems to override the user specific entry. 
I found moving the user specific entry so that is appears after the group entry fixes this. Maybe sudo parses the entire file and it uses the last matching entry? 

Changing your sudoers file to something similar to below should work.

# User privilege specification
root    ALL=(ALL) ALL
%admin  ALL=(ALL) ALL
howard  ALL=(ALL) NOPASSWD: ALL