Don't create modules' db classes if the module isn't marked for install

Since packstack introduced a DB_PW for each module, it started failing
when 1 of those modules is not marked for install because of some
missing variables in the CONF dict when formatting mysql.pp

This patch splits mysql configs per module in 3 different pp files and
includes them just when those modules are marked for install.

Fixes bz#903191

Change-Id: I7c1d86c7c80aa6e801f0ea68e2379abc01ab4967