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.
The cronjob is intalled where there is a ttl set.  In two deployments you can have the same packages, but in one you can have this cron and not in another: the reason is that one user set a TTL here.  This is something that I am not sure how to do with packages.