Allow user to specify another cookbook for my.cnf template source

have the template source for my.cnf be in another cookbook.

This allows me to modify the template to do whatever crazy stuff I need to do,
without significantly complicating the recipe, and leveraging all the great
things that the mysql cookbook already does.

To use set your own override

node['mysql']['cookbook'] = 'mycookbook'

and the recipe will fetch the my.cnf.erb from there.

My particular use case was that I want to disable innodb file per
table and a number of other changes.