Add time_to_live parameter into the barclamp

Add the parameter time_to_live (ceilometer.conf) into the barclamp,
so the user can decide when the meter lines are removed from the
database.

This patch was tested with a mongodb configuration.

This patch was made by Jiri (jsuchome). I only tested it and added
the parameter in the ceilometer.conf.erb template.
I checked this:
  https://docs.chef.io/resource_cron.html
In this case I need to create a proper cronjob, but this this template thing I can create the script itself and put it in the .daily / .weekly directory.  The user still can execute the command as root.

The other option is create a cronjob with the cron resource and create two scripts that are called by them.  What approach it the canonical here?