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.
Did we bump the revision for tex already, or was 12 the last one in stoney? (If no, we might want to bump to 15 or 20 to leave some room for migrations in stoney)