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.
Btw, re the `suse.de`, I think in this case, it's better to use a `crowbar` prefix since that's the project we're working on. And I'd just name the cron job: `crowbar-ceilometer-expirer`