Make it possible to specify own template.

Make it possible to specify own template.

Micah found an issue with usage of config_content: if you call template('...')
yourself and pass that on to config_content, then your template gets evaluated
without all of the variables. This means that you don't hava access to
blacklisted_packages, mail_recipient or mailonlyonerror.

To make it possible to use a different template while still having access to
those variables, let's make it possible to change the template name that we're
using.