Adds includedir back to templates

This adds includedir to the mysql templates, which allows overriding of
default my.cnf settings. Additionally, /etc/mysql/conf.d is created if it
doesn't already exist to prevent mysql from failing.

Change-Id: I1e051978b2363eaa9fd11abf1b107bd1e9c0edcb
Fixes: bug #1218613
Merge "Adds includedir back to templates"