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.
Repair database cronjob

The server recipe create and deploy a daily cronjob that removes
related elements in the database (according to time_to_live
parameter) and a weekly job that recreate the database in case
of a mongodb database.
Merge pull request #162 from aplanas/master

Add time_to_live parameter into the barclamp